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

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@gmail.com>

Series applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/63279eeb7f93abb1692573c26f1e038e1a87358b

cheers