Hi Alex,
Thanks for looking at this.
On 2020-12-10 10:12, Alexandru Elisei wrote:
Hi Marc,
On 12/10/20 8:30 AM, Marc Zyngier wrote:
We reset the guest's view of PMCR_EL0 unconditionally, based on
the host's view of this register. It is however legal for an
imnplementation not to provide any PMU, resulting in an UNDEF.
The obvious fix is to skip the reset of this shadow register
when no PMU is available, sidestepping the issue entirely.
If no PMU is available, the guest is not able to request
a virtual PMU anyway, so not doing nothing is the right thing
to do!
It is unlikely that this bug can hit any HW implementation
though, as they all provide a PMU. It has been found using nested
virt with the host KVM not implementing the PMU itself.
Fixes: ab9468340d2bc ("arm64: KVM: Add access handler for PMCR
register")
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
arch/arm64/kvm/sys_regs.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index bc15246775d0..6c64d010102b 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -923,6 +923,10 @@ static void reset_pmcr(struct kvm_vcpu *vcpu,
const struct sys_reg_desc *r)
{
u64 pmcr, val;
+ /* No PMU available, PMCR_EL0 may UNDEF... */
+ if (!kvm_arm_support_pmu_v3())
+ return;
+
reset_pmcr() is called from kvm_reset_vcpu()->kvm_reset_sys_regs().
Before calling kvm_reset_sys_regs(), kvm_reset_vcpu() returns -EINVAL
if the VCPU has the PMUv3 feature but the host doesn't have a PMU.
It looks to me like the undef can happen only when the VCPU feature
isn't set and the hardware doesn't have a PMU.
Which is exactly what I describe in the commit message (NV without PMU).
How about we change
the test to check for kvm_vcpu_has_pmu() to avoid executing the extra
instructions, which are not needed because the VM won't have a PMU?
I went down that road initially, and then realised that we need to
backport this as far back as 4.9 (the code was merged in 4.6).
I don't fancy backporting kvm_vcpu_has_pmu() and co...
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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