Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

[Isaku Yamahata]

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>