[PATCH 1/3] KVM: PPC: Book3S HV: Always save guest pmu for guest capable of nesting

Michael Ellerman patch-notifications at ellerman.id.au
Thu Jul 18 23:56:38 AEST 2019

On Wed, 2019-07-03 at 01:20:20 UTC, Suraj Jitindar Singh wrote:
> The performance monitoring unit (PMU) registers are saved on guest exit
> when the guest has set the pmcregs_in_use flag in its lppaca, if it
> exists, or unconditionally if it doesn't. If a nested guest is being
> run then the hypervisor doesn't, and in most cases can't, know if the
> pmu registers are in use since it doesn't know the location of the lppaca
> for the nested guest, although it may have one for its immediate guest.
> This results in the values of these registers being lost across nested
> guest entry and exit in the case where the nested guest was making use
> of the performance monitoring facility while it's nested guest hypervisor
> wasn't.
> Further more the hypervisor could interrupt a guest hypervisor between
> when it has loaded up the pmu registers and it calling H_ENTER_NESTED or
> between returning from the nested guest to the guest hypervisor and the
> guest hypervisor reading the pmu registers, in kvmhv_p9_guest_entry().
> This means that it isn't sufficient to just save the pmu registers when
> entering or exiting a nested guest, but that it is necessary to always
> save the pmu registers whenever a guest is capable of running nested guests
> to ensure the register values aren't lost in the context switch.
> Ensure the pmu register values are preserved by always saving their
> value into the vcpu struct when a guest is capable of running nested
> guests.
> This should have minimal performance impact however any impact can be
> avoided by booting a guest with "-machine pseries,cap-nested-hv=false"
> on the qemu commandline.
> Fixes: 95a6432ce903 "KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests"
> Signed-off-by: Suraj Jitindar Singh <sjitindarsingh at gmail.com>

Series applied to powerpc fixes, thanks.



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