Hi Alex, On Wed, Dec 1, 2021 at 8:09 AM Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote: > > Hi Reiji, > > On Wed, Dec 01, 2021 at 03:53:18PM +0000, Alexandru Elisei wrote: > > Hi Reiji, > > > > On Mon, Nov 29, 2021 at 09:32:02PM -0800, Reiji Watanabe wrote: > > > Hi Eric, > > > > > > On Thu, Nov 25, 2021 at 12:30 PM Eric Auger <eauger@xxxxxxxxxx> wrote: > > > > > > > > Hi Reiji, > > > > > > > > On 11/17/21 7:43 AM, Reiji Watanabe wrote: > > > > > When ID_AA64DFR0_EL1.PMUVER or ID_DFR0_EL1.PERFMON is 0xf, which > > > > > means IMPLEMENTATION DEFINED PMU supported, KVM unconditionally > > > > > expose the value for the guest as it is. Since KVM doesn't support > > > > > IMPLEMENTATION DEFINED PMU for the guest, in that case KVM should > > > > > exopse 0x0 (PMU is not implemented) instead. > > > > s/exopse/expose > > > > > > > > > > Change cpuid_feature_cap_perfmon_field() to update the field value > > > > > to 0x0 when it is 0xf. > > > > is it wrong to expose the guest with a Perfmon value of 0xF? Then the > > > > guest should not use it as a PMUv3? > > > > > > > is it wrong to expose the guest with a Perfmon value of 0xF? Then the > > > > guest should not use it as a PMUv3? > > > > > > For the value 0xf in ID_AA64DFR0_EL1.PMUVER and ID_DFR0_EL1.PERFMON, > > > Arm ARM says: > > > "IMPLEMENTATION DEFINED form of performance monitors supported, > > > PMUv3 not supported." > > > > > > Since the PMU that KVM supports for guests is PMUv3, 0xf shouldn't > > > be exposed to guests (And this patch series doesn't allow userspace > > > to set the fields to 0xf for guests). > > > > While it's true that a value of 0xf means that PMUv3 is not present (both > > KVM and the PMU driver handle it this way) this is an userspace visible > > change. > > > > Are you sure there isn't software in the wild that relies on this value > > being 0xf to detect that some non-Arm architected hardware is present? > > > > Since both 0 and 0xf are valid values that mean that PMUv3 is not present, > > I think it's best that both are kept. > > Sorry, somehow I managed to get myself confused and didn't realize that > this is only used by KVM. > > What I said above about the possibility of software existing that pokes IMP > DEF registers when PMUVer = 0xf is in fact a good argument for this patch, > because KVM injects an undefined exception when a guest tries to access > such registers. Thank you for your review. My understanding is the same as yours. Thanks, Reiji
