On Wed, Nov 06, 2024, Mario Limonciello wrote: > On 11/6/2024 09:48, Maxim Levitsky wrote: > > On Wed, 2024-11-06 at 07:15 -0800, Sean Christopherson wrote: > > > On Wed, Nov 06, 2024, Mario Limonciello wrote: > > > > On 11/6/2024 09:03, Sean Christopherson wrote: ... > > > > > > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c > > > > > > index 015971adadfc7..ecd42c2b3242e 100644 > > > > > > --- a/arch/x86/kernel/cpu/amd.c > > > > > > +++ b/arch/x86/kernel/cpu/amd.c > > > > > > @@ -924,6 +924,17 @@ static void init_amd_zen4(struct cpuinfo_x86 *c) > > > > > > { > > > > > > if (!cpu_has(c, X86_FEATURE_HYPERVISOR)) > > > > > > msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_SHARED_BTB_FIX_BIT); > > > > > > + > > > > > > + /* > > > > > > + * These Zen4 SoCs advertise support for virtualized VMLOAD/VMSAVE > > > > > > + * in some BIOS versions but they can lead to random host reboots. > > > > > > > > > > Uh, CPU bug? Erratum? > > > > > > > > BIOS bug. Those shouldn't have been advertised. > > > > Hi! > > > > My question is, why would AMD drop support intentionally for VLS on client machines? > > > > I understand that there might be a errata, and I don't object disabling the > > feature because of this. > > > > But hearing that 'These instructions aren't intended to be advertised' means that > > AMD intends to stop supporting virtualization on client systems or at least partially > > do so. > > Don't read into it too far. It's just a BIOS problem with those > instructions "specifically" on the processors indicated here. Other > processors (for example Zen 5 client processors) do correctly advertise > support where applicable. > > When they launched those bits weren't supposed to be set to indicate > support, but BIOS did set them. As you quite clearly call out below, this isn't simply a BIOS problem. > > That worries me. So far AMD was much better that Intel supporting most of the > > features across all of the systems which is very helpful in various scenarios, > > and this is very appreciated by the community. > > > > Speaking strictly personally here, as a AMD fan. > > > > Best regards,> Maxim Levitsky > > > > > > > > > > Why not? "but they can lead to random host reboots" is a description of the > > > symptom, not an explanation for why KVM is unable to use a feature that is > > > apparently support by the CPU. > > > > > > And if the CPU doesn't actually support virtualized VMLOAD/VMSAVE, then this is > > > a much bigger problem, because it means KVM is effectively giving the guest read > > > and write access to all of host memory. > > > > > > > > > I'm gathering that what supported means to you and what it means to me are > different things. Yes. And the distinction matters greatly in this case, because "VMLOAD/VMSAVE in the guest are broken" is *very* different than "VMLOAD/VMSAVE in the guest actually operate on SPAs, not GPAs". > "Architecturally" the instructions for virtualized VMLOAD/VMSAVE exist. Which means they're supported, but broken. > There are problems with them on these processors, and for that reason the > BIOS was not supposed to set those bits but it did. In other words, this a CPU bug. The kernel comment absolutely needs to reflect that. Passing this off as BIOS going rogue is misleading and confusing.