Re: [PATCH 8/9] KVM: x86: When guest set CR3, handle LAM bits semantics

Robert Hoo
2022-11-03T01:04:28Z

On Thu, 2022-11-03 at 00:05 +0300, Kirill A. Shutemov wrote:
> On Wed, Nov 02, 2022 at 03:29:10PM +0800, Robert Hoo wrote:
> > On Tue, 2022-11-01 at 05:04 +0300, Kirill A. Shutemov wrote:
> > ...
> > > > > > - if (cr3 != kvm_read_cr3(vcpu))
> > > > > > - kvm_mmu_new_pgd(vcpu, cr3);
> > > > > > + old_cr3 = kvm_read_cr3(vcpu);
> > > > > > + if (cr3 != old_cr3) {
> > > > > > + if ((cr3 ^ old_cr3) & CR3_ADDR_MASK) {
> > > > > > + kvm_mmu_new_pgd(vcpu, cr3 &
> > > > > > ~(X86_CR3_LAM_U48 |
> > > > > > + X86_CR3_LAM_U57));
> > > > > > + } else {
> > > > > > + /* Only LAM conf changes, no tlb flush
> > > > > > needed
> > > > > > */
> > > > > > + skip_tlb_flush = true;
> > > > >
> > > > > I'm not sure about this.
> > > > >
> > > > > Consider case when LAM_U48 gets enabled on 5-level paging
> > > > > machines.
> > > > > We may
> > > > > have valid TLB entries for addresses above 47-bit. It's kinda
> > > > > broken
> > > > > case,
> > > > > but seems valid from architectural PoV, no?
> > > >
> > > > You're right, thanks Kirill.
> > > >
> > > > I noticed in your Kernel enabling, because of this LAM_U48 and
> > > > LA_57
> > > > overlapping, you enabled LAM_U57 only for simplicity at this
> > > > moment. I
> > > > thought at that time, that this trickiness will be contained in
> > > > Kernel
> > > > layer, but now it turns out at least non-EPT KVM MMU is not
> > > > spared.
> > > > >
> > > > > I guess after enabling LAM, these entries will never match.
> > > > > But
> > > > > if
> > > > > LAM
> > > > > gets disabled again they will become active. Hm?
> > > > >
> > > > > Maybe just flush?
> > > >
> > > > Now we have 2 options
> > > > 1. as you suggested, just flush
> > > > 2. more precisely identify the case Guest.LA57 &&
> > > > (CR3.bit[62:61]
> > > > 00
> > > > -->10 switching), flush. (LAM_U57 bit take precedence over
> > > > LAM_U48,
> > > > from spec.)
> > > >
> > > > Considering CR3 change is relatively hot path, and tlb flush is
> > > > heavy,
> > > > I lean towards option 2. Your opinion?
> > >
> > > 11 in bits [62:61] is also considered LAM_U57. So your option 2
> > > is
> > > broken.
> >
> > Hi Kirill,
> >
> > When I came to cook v2 per your suggestion, i.e. leave it just
> > flush, I
> > pondered on the necessity on all the cases of the 2 bits (LAM_U48,
> > LAM_U57) flips.
> > Hold this: LAM_U57 (bit61) takes precedence over LAM_U48 (bit62).
> >
> > (0,0) --> {(0,1), (1,0), (1,1)}
> > (0,1) --> {(0,0), (1,0), (1,1)}
> > (1,0) --> {(0,0), (0,1), (1,1)}
> > (1,1) --> {(0,0), (1,0), (1,0)}
> >
> > Among all the 12 cases, only (0,0) --> (1,0) && 5-level paging on,
> > has
> > to flush tlb. Am I right? if so, would you still prefer
> > unconditionally
> > flush, just for 1/12 necessity? (if include 5-level/4-level
> > variations,
> > 1/24)
>
> I would keep it simple. We can always add optimization later if
> there's
> a workload that actually benefit from it. But I cannot imagine
> situation
> where enabling LAM is a hot path.
>
OK, I'm open to this.

I also notice that skip_tlb_flush is set when pcid_enabled && (CR3
& X86_CR3_PCID_NOFLUSH). Under this condition, do you think (0,0) -->
(1,0) need to flip it back to false?

int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
{
bool skip_tlb_flush = false;
unsigned long pcid = 0, old_cr3;
#ifdef CONFIG_X86_64
bool pcid_enabled = !!kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE);

if (pcid_enabled) {
skip_tlb_flush = cr3 & X86_CR3_PCID_NOFLUSH;