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.
}
