Re: [Xen-devel] [PATCH v2 8/9] x86/CPUID: drop INVPCID dependency on PCID
On 18/09/2019 10:10, Jan Beulich wrote: > On 17.09.2019 21:59, Andrew Cooper wrote: >> On 17/09/2019 07:17, Jan Beulich wrote: >>> PCID validly depends on LM, as it can be enabled in Long Mode only. >>> INVPCID, otoh, can be used not only without PCID enabled, but also >>> outside of Long Mode altogether. In both cases its functionality is >>> simply restricted to PCID 0, which is sort of expected as no other PCID >>> can be activated there. >>> >>> Signed-off-by: Jan Beulich >>> Reviewed-by: Roger Pau Monné >>> >>> --- a/xen/tools/gen-cpuid.py >>> +++ b/xen/tools/gen-cpuid.py >>> @@ -218,10 +218,6 @@ def crunch_numbers(state): >>> # >>> # SSE4_2: [POPCNT] >>> >>> -# The INVPCID instruction depends on PCID infrastructure being >>> -# available. >>> -PCID: [INVPCID], >> I take it you haven't tried running a shadow guest with this change in >> place. >> >> For !EPT, we clear ENABLE_INVPCID so the instruction will #UD within the >> guest. >> >> We could in principle allow the use of INVPCID with shadow guests, but >> that would involve teaching Xen how to cope with VMEXIT_REASON_INVPCID >> (which is predicated on INVLPG exiting) and wire the flush back into the >> shadow logic. >> >> Perhaps the comment could be a little clearer, but it is no accident >> that dependency is in place. > I'm afraid I don't follow: Both features are marked H in the > public header, so I don't see how shadow mode considerations > apply here at all. Oh. Of course. I'm not confused as to why that was present to being with. Acked-by: Andrew Cooper ___ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
Re: [Xen-devel] [PATCH v2 8/9] x86/CPUID: drop INVPCID dependency on PCID
On 17.09.2019 21:59, Andrew Cooper wrote: > On 17/09/2019 07:17, Jan Beulich wrote: >> PCID validly depends on LM, as it can be enabled in Long Mode only. >> INVPCID, otoh, can be used not only without PCID enabled, but also >> outside of Long Mode altogether. In both cases its functionality is >> simply restricted to PCID 0, which is sort of expected as no other PCID >> can be activated there. >> >> Signed-off-by: Jan Beulich >> Reviewed-by: Roger Pau Monné >> >> --- a/xen/tools/gen-cpuid.py >> +++ b/xen/tools/gen-cpuid.py >> @@ -218,10 +218,6 @@ def crunch_numbers(state): >> # >> # SSE4_2: [POPCNT] >> >> -# The INVPCID instruction depends on PCID infrastructure being >> -# available. >> -PCID: [INVPCID], > > I take it you haven't tried running a shadow guest with this change in > place. > > For !EPT, we clear ENABLE_INVPCID so the instruction will #UD within the > guest. > > We could in principle allow the use of INVPCID with shadow guests, but > that would involve teaching Xen how to cope with VMEXIT_REASON_INVPCID > (which is predicated on INVLPG exiting) and wire the flush back into the > shadow logic. > > Perhaps the comment could be a little clearer, but it is no accident > that dependency is in place. I'm afraid I don't follow: Both features are marked H in the public header, so I don't see how shadow mode considerations apply here at all. Jan ___ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
Re: [Xen-devel] [PATCH v2 8/9] x86/CPUID: drop INVPCID dependency on PCID
On 17/09/2019 07:17, Jan Beulich wrote: > PCID validly depends on LM, as it can be enabled in Long Mode only. > INVPCID, otoh, can be used not only without PCID enabled, but also > outside of Long Mode altogether. In both cases its functionality is > simply restricted to PCID 0, which is sort of expected as no other PCID > can be activated there. > > Signed-off-by: Jan Beulich > Reviewed-by: Roger Pau Monné > > --- a/xen/tools/gen-cpuid.py > +++ b/xen/tools/gen-cpuid.py > @@ -218,10 +218,6 @@ def crunch_numbers(state): > # > # SSE4_2: [POPCNT] > > -# The INVPCID instruction depends on PCID infrastructure being > -# available. > -PCID: [INVPCID], I take it you haven't tried running a shadow guest with this change in place. For !EPT, we clear ENABLE_INVPCID so the instruction will #UD within the guest. We could in principle allow the use of INVPCID with shadow guests, but that would involve teaching Xen how to cope with VMEXIT_REASON_INVPCID (which is predicated on INVLPG exiting) and wire the flush back into the shadow logic. Perhaps the comment could be a little clearer, but it is no accident that dependency is in place. ~Andrew ___ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
