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

Robert Hoo
2022-11-02T07:29:16Z

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)

>
> And I don't buy argument about hot path: the case we talking about is
> about enabling/disabling LAM with constant PGD. It's not hot path by
> any
> mean.
>
> Let's not be fancy. Just flush TLB.
>