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

[Lai Jiangshan]

On 2021/5/28 00:13, Sean Christopherson wrote:
> +Maxim - A proper fix for this bug might fix your shadow paging + win10 boot
>           issue, this also affects the KVM_REQ_HV_TLB_FLUSH used for HyperV PV
> 	 flushing.
>
> On Thu, May 27, 2021, Paolo Bonzini wrote:
> > On 27/05/21 04:39, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > >
> > > For KVM_VCPU_FLUSH_TLB used in kvm_flush_tlb_multi(), the guest expects
> > > the hypervisor do the operation that equals to native_flush_tlb_global()
> > > or invpcid_flush_all() in the specified guest CPU.
> > >
> > > When TDP is enabled, there is no problem to just flush the hardware
> > > TLB of the specified guest CPU.
> > >
> > > But when using shadowpaging, the hypervisor should have to sync the
> > > shadow pagetable at first before flushing the hardware TLB so that
> > > it can truely emulate the operation of invpcid_flush_all() in guest.
> >
> > Can you explain why?
>
> KVM's unsync logic hinges on guest TLB flushes.  For page permission modifications
> that require a TLB flush to take effect, e.g. making a writable page read-only,
> KVM waits until the guest explicitly does said flush to propagate the changes to
> the shadow page tables.  E.g. failure to sync PTEs could result in a read-only 4k
> page being writable when the guest expects it to be read-only.
>
> > Also it is simpler to handle this in kvm_vcpu_flush_tlb_guest, using "if
> > (tdp_enabled).  This provides also a single, good place to add a comment
> > with the explanation of what invalid entries KVM_REQ_RELOAD is presenting.
>
> Ya.
>
> KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely
> offset the performance gains of the paravirtualized flush >
> And making a request won't work without revamping the order of request handling
> in vcpu_enter_guest(), e.g. KVM_REQ_MMU_RELOAD and KVM_REQ_MMU_SYNC are both
> serviced before KVM_REQ_STEAL_UPDATE.

Yes, it just fixes the said problem in the simplest way.
I copied KVM_REQ_MMU_RELOAD from kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).
(If the guest is not preempted, it will call invpcid_flush_all() and will be handled
by this way)


The improvement code will go later, and will not be backported.
The proper way to flush guest is to use code in

https://lore.kernel.org/lkml/20210525213920.3340-1-jiangshanlai@xxxxxxxxx/
as:
+		kvm_mmu_sync_roots(vcpu);
+		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); //or just call flush_current directly
+		for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
+			vcpu->arch.mmu->prev_roots[i].need_sync = true;

If need_sync patch is not accepted, we can just use kvm_mmu_sync_roots(vcpu)
to keep the current pagetable and use kvm_mmu_free_roots() to free all the other
roots in prev_roots.



> Cleaning up and documenting the MMU related requests is on my todo list, but the
> immediate fix should be tiny and I can do my cleanups on top.
>
> 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);

it doesn't handle prev_roots which are also needed as
shown in kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).

>          static_call(kvm_x86_tlb_flush_guest)(vcpu);

For tdp_enabled, I think it is better to use kvm_x86_tlb_flush_current()
to make it consistent with other shadowpage code.

>   }
>