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X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: l5oNoZ4t6S+brlPyA52XsEu0D2mmNCsB4IfZvl99AM4e5Z875hK0nqgNpVKmihpaimVb5q9NyQEqYotcZoNGYg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR03MB5465 On Mon, May 23, 2022 at 10:12:55AM +0200, Jan Beulich wrote: > On 20.05.2022 16:58, 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", > > Which only rather new guest kernels will know to look for. Hence ... > > > 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. > > ... while in principle I agree with this, I see a practical issue. I think it should be fine to expose no model-specific LBR support in PERF_CAPABILITIES, but we shouldn't change the behavior of DEBUGCTLMSR.LBR exposed to guests if the underlying platform has model-specific LBRs and those are known to Xen. That way old guest kernels that ignore PERF_CAPABILITIES.LBR_FORMAT will continue to work, while newish kernels that check the format will avoid using LBRs. In case we introduce a guest config option to enable LBR, should we then expose the native LBR format in PERF_CAPABILITIES? Or would it be better to just keep the current model and not expose PERF_CAPABILITIES at all (and don't report PDCM in CPUID in that case). Thanks, Roger.