On Mon, Mar 25, 2024 at 07:55:04PM +0000, "Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx> wrote: > On Mon, 2024-03-25 at 12:05 -0700, Isaku Yamahata wrote: > > Right, the guest has to accept it on VE. If the unmap was intentional by guest, > > that's fine. The unmap is unintentional (with vMTRR), the guest doesn't expect > > VE with the GPA. > > > > > > > But, I guess we should punt to userspace is the guest tries to use > > > MTRRs, not that userspace can handle it happening in a TD... But it > > > seems cleaner and safer then skipping zapping some pages inside the > > > zapping code. > > > > > > I'm still not sure if I understand the intention and constraints fully. > > > So please correct. This (the skipping the zapping for some operations) > > > is a theoretical correctness issue right? It doesn't resolve a TD > > > crash? > > > > For lapic, it's safe guard. Because TDX KVM disables APICv with > > APICV_INHIBIT_REASON_TDX, apicv won't call kvm_zap_gfn_range(). > Ah, I see it: > https://lore.kernel.org/lkml/38e2f8a77e89301534d82325946eb74db3e47815.1708933498.git.isaku.yamahata@xxxxxxxxx/ > > Then it seems a warning would be more appropriate if we are worried there might be a way to still > call it. If we are confident it can't, then we can just ignore this case. > > > > > For MTRR, the purpose is to make the guest boot (without the guest kernel > > command line like clearcpuid=mtrr) . > > If we can assume the guest won't touch MTRR registers somehow, KVM can return an > > error to TDG.VP.VMCALL<RDMSR, WRMSR>(MTRR registers). So it doesn't call > > kvm_zap_gfn_range(). Or we can use KVM_EXIT_X86_{RDMSR, WRMSR} as you suggested. > > My understanding is that Sean prefers to exit to userspace when KVM can't handle something, versus > making up behavior that keeps known guests alive. So I would think we should change this patch to > only be about not using the zapping roots optimization. Then a separate patch should exit to > userspace on attempt to use MTRRs. And we ignore the APIC one. > > This is trying to guess what maintainers would want here. I'm less sure what Paolo prefers. When we hit KVM_MSR_FILTER, the current implementation ignores it and makes it error to guest. Surely we should make it KVM_EXIT_X86_{RDMSR, WRMSR}, instead. It's aligns with the existing implementation(default VM and SW-protected) and more flexible. -- Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
