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

[Lai Jiangshan <laijs@linux.alibaba.com>]

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?

If CR4.PCIDE = 1 and bit 63 of the instruction’s source operand is 0, the instruction invalidates all TLB entries 
associated with the PCID specified in bits 11:0 of the instruction’s source operand except those for global pages. It 
also invalidates all entries in all paging-structure caches associated with that PCID. It is not required to invalidate 
entries in the TLBs and paging-structure caches that are associated with other PCIDs.

> 
> Lai, if it's ok with you, I'll massage this patch as discussed and fold it into
> a larger series to fix the other bugs and do additional cleanup/improvements.
> 
>>> I believe the minimal fix is:
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 81ab3b8f22e5..b0072063f9bf 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -3072,6 +3072,9 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
>>>   static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
>>>   {
>>>          ++vcpu->stat.tlb_flush;
>>> +
>>> +       if (!tdp_enabled)
>>> +               kvm_mmu_sync_roots(vcpu);
>>>          static_call(kvm_x86_tlb_flush_guest)(vcpu);
>>>   }