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X-MS-Exchange-CrossTenant-UserPrincipalName: nYC1pydRSPvw5/KxZb7yJEKhTP0SbDMMwfKcGIkgzmqUIdp4s52wfgR8tXXQNYGoe1Hj8bANSPjM4YKwb5UiTQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ0PR03MB5454 On Fri, May 20, 2022 at 02:58:01PM +0000, Andrew Cooper wrote: > On 20/05/2022 15:19, Jan Beulich wrote: > > On 20.05.2022 16:10, Andrew Cooper wrote: > >> On 20/05/2022 14:37, Roger Pau Monne wrote: > >>> --- a/xen/include/public/arch-x86/cpufeatureset.h > >>> +++ b/xen/include/public/arch-x86/cpufeatureset.h > >>> @@ -135,7 +135,7 @@ XEN_CPUFEATURE(SSSE3, 1*32+ 9) /*A Supplemental Streaming SIMD Extensio > >>> XEN_CPUFEATURE(FMA, 1*32+12) /*A Fused Multiply Add */ > >>> XEN_CPUFEATURE(CX16, 1*32+13) /*A CMPXCHG16B */ > >>> XEN_CPUFEATURE(XTPR, 1*32+14) /* Send Task Priority Messages */ > >>> -XEN_CPUFEATURE(PDCM, 1*32+15) /* Perf/Debug Capability MSR */ > >>> +XEN_CPUFEATURE(PDCM, 1*32+15) /*S Perf/Debug Capability MSR */ > >> This is the bit which requires more toolstack logic to safely enable.  > >> Using 's' for off-by-default is fine if we want to get the series in now. > >> > >> But before we expose the MSR generally, we need to: > >> > >> 1) Put the configuration in msr_policy so the toolstack can reason about it > >> 2) Reject migration attempts to destinations where the LBR format changes > > Since this could be quite restrictive, and since people needing to know > > they need to hide this feature for migration to work, I guess this would > > further want qualifying by "did the guest actually use LBRs so far"? > > In practice, it's every major generation ("tock" on Intel's old model), > so isn't actually limiting the kinds of heterogeneous setups used in > production.  (Migration gets steadily less stable the further apart the > two CPUs are.) > > As to dynamic, no - that would be a security bug in a cloud scenario, > because there must not be anything the guest can do to interfere with > the manageability. > > Use of LBR is rare, as demonstrated by the fact that noone has > complained about the fact that migrating such a VM will malfunction. > > As we now have a way of reporting "no model-specific LBR", I'm tempted > to suggest that VMs get no LBR by default, and someone wanting LBR has > to opt in, which is also an explicit agreement to the migration limitation. I did also consider exposing "no model-specific LBR" in PERF_CAPABILITIES unconditionally, but I was worried whether that would break existing setups. If we think that providing an option to expose the native LBR format in PERF_CAPABILITIES is fine that could be a sensible solution IMO. Thanks, Roger.