[PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow_pfn

[Yu Zhang]

On Tue, Jul 04, 2023 at 04:50:50PM +0900, David Stevens wrote:
> From: David Stevens <stevensd at chromium.org>
> 
> Stop passing FOLL_GET to __kvm_follow_pfn. This allows the host to map
> memory into the guest that is backed by un-refcounted struct pages - for
> example, higher order non-compound pages allocated by the amdgpu driver
> via ttm_pool_alloc_page.

I guess you mean the tail pages of the higher order non-compound pages?
And as to the head page, it is said to be set to one coincidentally[*],
and shall not be considered as refcounted.  IIUC, refcount of this head
page will be increased and decreased soon in hva_to_pfn_remapped(), so
this may not be a problem(?). But treating this head page differently,
as a refcounted one(e.g., to set the A/D flags), is weired. 

Or maybe I missed some context, e.g., can the head page be allocted to
guest at all? 


> 
> The bulk of this change is tracking the is_refcounted_page flag so that
> non-refcounted pages don't trigger page_count() == 0 warnings. This is
> done by storing the flag in an unused bit in the sptes.

Also, maybe we should mention this only works on x86-64. 

> 
> Signed-off-by: David Stevens <stevensd at chromium.org>
> ---
>  arch/x86/kvm/mmu/mmu.c          | 44 +++++++++++++++++++++------------
>  arch/x86/kvm/mmu/mmu_internal.h |  1 +
>  arch/x86/kvm/mmu/paging_tmpl.h  |  9 ++++---
>  arch/x86/kvm/mmu/spte.c         |  4 ++-
>  arch/x86/kvm/mmu/spte.h         | 12 ++++++++-
>  arch/x86/kvm/mmu/tdp_mmu.c      | 22 ++++++++++-------
>  6 files changed, 62 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e44ab512c3a1..b1607e314497 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c

...

> @@ -2937,6 +2943,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
>  	bool host_writable = !fault || fault->map_writable;
>  	bool prefetch = !fault || fault->prefetch;
>  	bool write_fault = fault && fault->write;
> +	bool is_refcounted = !fault || fault->is_refcounted_page;

Just wonder, what if a non-refcounted page is prefetched?  Or is it possible in
practice?

...
>  
> @@ -883,7 +884,7 @@ static gpa_t FNAME(gva_to_gpa)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>   */
>  static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int i)
>  {
> -	bool host_writable;
> +	bool host_writable, is_refcounted;
>  	gpa_t first_pte_gpa;
>  	u64 *sptep, spte;
>  	struct kvm_memory_slot *slot;
> @@ -940,10 +941,12 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
>  	sptep = &sp->spt[i];
>  	spte = *sptep;
>  	host_writable = spte & shadow_host_writable_mask;
> +	// TODO: is this correct?
> +	is_refcounted = spte & SPTE_MMU_PAGE_REFCOUNTED;
>  	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
>  	make_spte(vcpu, sp, slot, pte_access, gfn,
>  		  spte_to_pfn(spte), spte, true, false,
> -		  host_writable, &spte);
> +		  host_writable, is_refcounted, &spte);

Could we restrict that a non-refcounted page shall not be used as shadow page?

[*] https://lore.kernel.org/all/8caf3008-dcf3-985a-631e-e019b277c6f0@amd.com/

B.R.
Yu