[PATCH RFC 1/1] KVM: PPC: Book3S HV: pack VCORE IDs to access full VCPU ID space
Sam Bobroff
sam.bobroff at au1.ibm.com
Tue Apr 24 13:19:15 AEST 2018
On Mon, Apr 23, 2018 at 11:06:35AM +0200, Cédric Le Goater wrote:
> On 04/16/2018 06:09 AM, David Gibson wrote:
> > On Thu, Apr 12, 2018 at 05:02:06PM +1000, Sam Bobroff wrote:
> >> It is not currently possible to create the full number of possible
> >> VCPUs (KVM_MAX_VCPUS) on Power9 with KVM-HV when the guest uses less
> >> threads per core than it's core stride (or "VSMT mode"). This is
> >> because the VCORE ID and XIVE offsets to grow beyond KVM_MAX_VCPUS
> >> even though the VCPU ID is less than KVM_MAX_VCPU_ID.
> >>
> >> To address this, "pack" the VCORE ID and XIVE offsets by using
> >> knowledge of the way the VCPU IDs will be used when there are less
> >> guest threads per core than the core stride. The primary thread of
> >> each core will always be used first. Then, if the guest uses more than
> >> one thread per core, these secondary threads will sequentially follow
> >> the primary in each core.
> >>
> >> So, the only way an ID above KVM_MAX_VCPUS can be seen, is if the
> >> VCPUs are being spaced apart, so at least half of each core is empty
> >> and IDs between KVM_MAX_VCPUS and (KVM_MAX_VCPUS * 2) can be mapped
> >> into the second half of each core (4..7, in an 8-thread core).
> >>
> >> Similarly, if IDs above KVM_MAX_VCPUS * 2 are seen, at least 3/4 of
> >> each core is being left empty, and we can map down into the second and
> >> third quarters of each core (2, 3 and 5, 6 in an 8-thread core).
> >>
> >> Lastly, if IDs above KVM_MAX_VCPUS * 4 are seen, only the primary
> >> threads are being used and 7/8 of the core is empty, allowing use of
> >> the 1, 3, 5 and 7 thread slots.
> >>
> >> (Strides less than 8 are handled similarly.)
> >>
> >> This allows the VCORE ID or offset to be calculated quickly from the
> >> VCPU ID or XIVE server numbers, without access to the VCPU structure.
> >>
> >> Signed-off-by: Sam Bobroff <sam.bobroff at au1.ibm.com>
> >> ---
> >> Hello everyone,
> >>
> >> I've tested this on P8 and P9, in lots of combinations of host and guest
> >> threading modes and it has been fine but it does feel like a "tricky"
> >> approach, so I still feel somewhat wary about it.
>
> Have you done any migration ?
No, but I will :-)
> >> I've posted it as an RFC because I have not tested it with guest native-XIVE,
> >> and I suspect that it will take some work to support it.
>
> The KVM XIVE device will be different for XIVE exploitation mode, same structures
> though. I will send a patchset shortly.
Great. This is probably where conflicts between the host and guest
numbers will show up. (See dwg's question below.)
> >> arch/powerpc/include/asm/kvm_book3s.h | 19 +++++++++++++++++++
> >> arch/powerpc/kvm/book3s_hv.c | 14 ++++++++++----
> >> arch/powerpc/kvm/book3s_xive.c | 9 +++++++--
> >> 3 files changed, 36 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> >> index 376ae803b69c..1295056d564a 100644
> >> --- a/arch/powerpc/include/asm/kvm_book3s.h
> >> +++ b/arch/powerpc/include/asm/kvm_book3s.h
> >> @@ -368,4 +368,23 @@ extern int kvmppc_h_logical_ci_store(struct kvm_vcpu *vcpu);
> >> #define SPLIT_HACK_MASK 0xff000000
> >> #define SPLIT_HACK_OFFS 0xfb000000
> >>
> >> +/* Pack a VCPU ID from the [0..KVM_MAX_VCPU_ID) space down to the
> >> + * [0..KVM_MAX_VCPUS) space, while using knowledge of the guest's core stride
> >> + * (but not it's actual threading mode, which is not available) to avoid
> >> + * collisions.
> >> + */
> >> +static inline u32 kvmppc_pack_vcpu_id(struct kvm *kvm, u32 id)
> >> +{
> >> + const int block_offsets[MAX_SMT_THREADS] = {0, 4, 2, 6, 1, 5, 3, 7};
> >
> > I'd suggest 1,3,5,7 at the end rather than 1,5,3,7 - accomplishes
> > roughly the same thing, but I think makes the pattern more obvious.
OK.
> >> + int stride = kvm->arch.emul_smt_mode > 1 ?
> >> + kvm->arch.emul_smt_mode : kvm->arch.smt_mode;
> >
> > AFAICT from BUG_ON()s etc. at the callsites, kvm->arch.smt_mode must
> > always be 1 when this is called, so the conditional here doesn't seem
> > useful.
Ah yes, right. (That was an older version when I was thinking of using
it for P8 as well but that didn't seem to be a good idea.)
> >> + int block = (id / KVM_MAX_VCPUS) * (MAX_SMT_THREADS / stride);
> >> + u32 packed_id;
> >> +
> >> + BUG_ON(block >= MAX_SMT_THREADS);
> >> + packed_id = (id % KVM_MAX_VCPUS) + block_offsets[block];
> >> + BUG_ON(packed_id >= KVM_MAX_VCPUS);
> >> + return packed_id;
> >> +}
> >
> > It took me a while to wrap my head around the packing function, but I
> > think I got there in the end. It's pretty clever.
Thanks, I'll try to add a better description as well :-)
> > One thing bothers me, though. This certainly packs things under
> > KVM_MAX_VCPUS, but not necessarily under the actual number of vcpus.
> > e.g. KVM_MAC_VCPUS==16, 8 vcpus total, stride 8, 2 vthreads/vcore (as
> > qemu sees it), gives both unpacked IDs (0, 1, 8, 9, 16, 17, 24, 25)
> > and packed ids of (0, 1, 8, 9, 4, 5, 12, 13) - leaving 2, 3, 6, 7
> > etc. unused.
That's right. The property it provides is that all the numbers are under
KVM_MAX_VCPUS (which, see below, is the size of the fixed areas) not
that they are sequential.
> > So again, the question is what exactly are these remapped IDs useful
> > for. If we're indexing into a bare array of structures of size
> > KVM_MAX_VCPUS then we're *already* wasting a bunch of space by having
> > more entries than vcpus. If we're indexing into something sparser,
> > then why is the remapping worthwhile?
Well, here's my thinking:
At the moment, kvm->vcores[] and xive->vp_base are both sized by NR_CPUS
(via KVM_MAX_VCPUS and KVM_MAX_VCORES which are both NR_CPUS). This is
enough space for the maximum number of VCPUs, and some space is wasted
when the guest uses less than this (but KVM doesn't know how many will
be created, so we can't do better easily). The problem is that the
indicies overflow before all of those VCPUs can be created, not that
more space is needed.
We could fix the overflow by expanding these areas to KVM_MAX_VCPU_ID
but that will use 8x the space we use now, and we know that no more than
KVM_MAX_VCPUS will be used so all this new space is basically wasted.
So remapping seems better if it will work. (Ben H. was strongly against
wasting more XIVE space if possible.)
In short, remapping provides a way to allow the guest to create it's full set
of VCPUs without wasting any more space than we do currently, without
having to do something more complicated like tracking used IDs or adding
additional KVM CAPs.
> >> +
> >> #endif /* __ASM_KVM_BOOK3S_H__ */
> >> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> >> index 9cb9448163c4..49165cc90051 100644
> >> --- a/arch/powerpc/kvm/book3s_hv.c
> >> +++ b/arch/powerpc/kvm/book3s_hv.c
> >> @@ -1762,7 +1762,7 @@ static int threads_per_vcore(struct kvm *kvm)
> >> return threads_per_subcore;
> >> }
> >>
> >> -static struct kvmppc_vcore *kvmppc_vcore_create(struct kvm *kvm, int core)
> >> +static struct kvmppc_vcore *kvmppc_vcore_create(struct kvm *kvm, int id)
> >> {
> >> struct kvmppc_vcore *vcore;
> >>
> >> @@ -1776,7 +1776,7 @@ static struct kvmppc_vcore *kvmppc_vcore_create(struct kvm *kvm, int core)
> >> init_swait_queue_head(&vcore->wq);
> >> vcore->preempt_tb = TB_NIL;
> >> vcore->lpcr = kvm->arch.lpcr;
> >> - vcore->first_vcpuid = core * kvm->arch.smt_mode;
> >> + vcore->first_vcpuid = id;
> >> vcore->kvm = kvm;
> >> INIT_LIST_HEAD(&vcore->preempt_list);
> >>
> >> @@ -1992,12 +1992,18 @@ static struct kvm_vcpu *kvmppc_core_vcpu_create_hv(struct kvm *kvm,
> >> mutex_lock(&kvm->lock);
> >> vcore = NULL;
> >> err = -EINVAL;
> >> - core = id / kvm->arch.smt_mode;
> >> + if (cpu_has_feature(CPU_FTR_ARCH_300)) {
> >> + BUG_ON(kvm->arch.smt_mode != 1);
> >> + core = kvmppc_pack_vcpu_id(kvm, id);
> >> + } else {
> >> + core = id / kvm->arch.smt_mode;
> >> + }
> >> if (core < KVM_MAX_VCORES) {
> >> vcore = kvm->arch.vcores[core];
> >> + BUG_ON(cpu_has_feature(CPU_FTR_ARCH_300) && vcore);
> >> if (!vcore) {
> >> err = -ENOMEM;
> >> - vcore = kvmppc_vcore_create(kvm, core);
> >> + vcore = kvmppc_vcore_create(kvm, id & ~(kvm->arch.smt_mode - 1));
> >> kvm->arch.vcores[core] = vcore;
> >> kvm->arch.online_vcores++;
> >> }
> >> diff --git a/arch/powerpc/kvm/book3s_xive.c b/arch/powerpc/kvm/book3s_xive.c
> >> index f9818d7d3381..681dfe12a5f3 100644
> >> --- a/arch/powerpc/kvm/book3s_xive.c
> >> +++ b/arch/powerpc/kvm/book3s_xive.c
> >> @@ -317,6 +317,11 @@ static int xive_select_target(struct kvm *kvm, u32 *server, u8 prio)
> >> return -EBUSY;
> >> }
> >>
> >> +static u32 xive_vp(struct kvmppc_xive *xive, u32 server)
> >> +{
> >> + return xive->vp_base + kvmppc_pack_vcpu_id(xive->kvm, server);
> >> +}
> >> +
> >
> > I'm finding the XIVE indexing really baffling. There are a bunch of
> > other places where the code uses (xive->vp_base + NUMBER) directly.
Ugh, yes. It looks like I botched part of my final cleanup and all the
cases you saw in kvm/book3s_xive.c should have been replaced with a call to
xive_vp(). I'll fix it and sorry for the confusion.
> This links the QEMU vCPU server NUMBER to a XIVE virtual processor number
> in OPAL. So we need to check that all used NUMBERs are, first, consistent
> and then, in the correct range.
Right. My approach was to allow XIVE to keep using server numbers that
are equal to VCPU IDs, and just pack down the ID before indexing into
the vp_base area.
> > If those are host side references, I guess they don't need updates for
> > this.
These are all guest side references.
> > But if that's the case, then how does indexing into the same array
> > with both host and guest server numbers make sense?
Right, it doesn't make sense to mix host and guest server numbers when
we're remapping only the guest ones, but in this case (without native
guest XIVE support) it's just guest ones.
> yes. VPs are allocated with KVM_MAX_VCPUS :
>
> xive->vp_base = xive_native_alloc_vp_block(KVM_MAX_VCPUS);
>
> but
>
> #define KVM_MAX_VCPU_ID (threads_per_subcore * KVM_MAX_VCORES)
>
> WE would need to change the allocation of the VPs I guess.
Yes, this is one of the structures that overflow if we don't pack the IDs.
> >> static u8 xive_lock_and_mask(struct kvmppc_xive *xive,
> >> struct kvmppc_xive_src_block *sb,
> >> struct kvmppc_xive_irq_state *state)
> >> @@ -1084,7 +1089,7 @@ int kvmppc_xive_connect_vcpu(struct kvm_device *dev,
> >> pr_devel("Duplicate !\n");
> >> return -EEXIST;
> >> }
> >> - if (cpu >= KVM_MAX_VCPUS) {
> >> + if (cpu >= KVM_MAX_VCPU_ID) {>>
> >> pr_devel("Out of bounds !\n");
> >> return -EINVAL;
> >> }
> >> @@ -1098,7 +1103,7 @@ int kvmppc_xive_connect_vcpu(struct kvm_device *dev,
> >> xc->xive = xive;
> >> xc->vcpu = vcpu;
> >> xc->server_num = cpu;
> >> - xc->vp_id = xive->vp_base + cpu;
> >> + xc->vp_id = xive_vp(xive, cpu);
> >> xc->mfrr = 0xff;
> >> xc->valid = true;
> >>
> >
>
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