Re: [PATCH] KVM: X86: fix tlb_flush_guest()

[Sean Christopherson]

On Wed, Jun 02, 2021, Sean Christopherson wrote:
> On Fri, May 28, 2021, Lai Jiangshan wrote:
> > 
> > 
> > On 2021/5/28 03:28, Sean Christopherson wrote:
> > > On Thu, May 27, 2021, Sean Christopherson wrote:
> > > > > KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely
> > > > > offset the performance gains of the paravirtualized flush.
> > > 
> > > Argh, I take that back.  The PV KVM_VCPU_FLUSH_TLB flag doesn't distinguish
> > > between flushing a specific mm and flushing the entire TLB.  The HyperV usage
> > > (via KVM_REQ) also throws everything into a single bucket.  A full RELOAD still
> > > isn't necessary as KVM just needs to sync all roots, not blast them away.  For
> > > previous roots, KVM doesn't have a mechanism to defer the sync, so the immediate
> > > fix will need to unload those roots.
> > > 
> > > And looking at KVM's other flows, __kvm_mmu_new_pgd() and kvm_set_cr3() are also
> > > broken with respect to previous roots.  E.g. if the guest does a MOV CR3 that
> > > flushes the entire TLB, followed by a MOV CR3 with PCID_NOFLUSH=1, KVM will fail
> > > to sync the MMU on the second flush even though the guest can technically rely
> > > on the first MOV CR3 to have synchronized any previous changes relative to the
> > > fisrt MOV CR3.
> > 
> > Could you elaborate the problem please?
> > When can a MOV CR3 that needs to flush the entire TLB if PCID is enabled?
> 
> Scratch that, I was wrong.  The SDM explicitly states that other PCIDs don't
> need to be flushed if CR4.PCIDE=1.

*sigh*

I was partially right.  If the guest does

  1: MOV    B, %rax
     MOV %rax, %cr3

  2: <modify PTEs in B>

  3: MOV    A, %rax
     MOV %rax, %cr3
 
  4: MOV    B, %rax
     BTS  $63, %rax
     MOV %rax, %cr3

where A and B are CR3 values with the same PCID, then KVM will fail to sync B at
step (4) due to PCID_NOFLUSH, even though the guest can technically rely on
its modifications at step (2) to become visible at step (3) when the PCID is
flushed on CR3 load.

So it's not a full TLB flush, rather a flush of the PCID, which can theoretically
impact previous CR3 values.