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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 





On 2021/5/28 08:26, Sean Christopherson wrote:
On Fri, May 28, 2021, Lai Jiangshan wrote:

On 2021/5/28 00:13, Sean Christopherson wrote:
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 problem is that record_steal_time() is called after KVM_REQ_MMU_RELOAD
in vcpu_enter_guest() and so the reload request won't be recognized until the
next VM-Exit.  It works for kvm_handle_invpcid() because vcpu_enter_guest() is
guaranteed to run between the invcpid code and VM-Enter.

Kvm will recheck the request before VM-enter.
See kvm_vcpu_exit_request().
It won't lost any request, it just causes a additional iteration.


The improvement code will go later, and will not be backported.

I would argue that introducing a potential performance regression is in itself a
bug.  IMO, going straight to kvm_mmu_sync_roots() is not high risk.

I think we can introduce a kvm_mmu_sync_roots_all() which syncs current
root and handles prev_roots (mark need_sync or drop) as cleanup/preparation patch
and then fix the problem.

Do we need to backport the cleanup/preparation patch or just backport the way
with KVM_REQ_MMU_RELOAD?



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.

I like the idea, I just haven't gotten around to reviewing that patch yet.

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).

Ya, I belated realized this :-)

          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.

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


   }




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux