github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-06-03 20:14:06 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

KVM: SVM: Add a comment to explain why avic_vcpu_blocking() ignores IRQ blocking

Browse Source 
Add a comment to explain why KVM clears IsRunning when putting a vCPU,
even though leaving IsRunning=1 would be ok from a functional perspective.
Per Maxim's experiments, a misbehaving VM could spam the AVIC doorbell so
fast as to induce a 50%+ loss in performance.

Link: https://lore.kernel.org/all/8d7e0d0391df4efc7cb28557297eb2ec9904f1e5.camel@redhat.com
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Acked-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20250611224604.313496-22-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2025-06-11 15:45:23 -07:00
parent 6737557442
commit 52d826c9e5
1 changed files with 18 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

31
arch/x86/kvm/svm/avic.c
View File

@ -1122,19 +1122,24 @@ void avic_vcpu_blocking(struct kvm_vcpu *vcpu)
if (!kvm_vcpu_apicv_active(vcpu))
return;
/*
* Unload the AVIC when the vCPU is about to block, _before_
* the vCPU actually blocks.
*
* Any IRQs that arrive before IsRunning=0 will not cause an
* incomplete IPI vmexit on the source, therefore vIRR will also
* be checked by kvm_vcpu_check_block() before blocking. The
* memory barrier implicit in set_current_state orders writing
* IsRunning=0 before reading the vIRR. The processor needs a
* matching memory barrier on interrupt delivery between writing
* IRR and reading IsRunning; the lack of this barrier might be
* the cause of errata #1235).
*/
/*
* Unload the AVIC when the vCPU is about to block, _before_ the vCPU
* actually blocks.
*
* Note, any IRQs that arrive before IsRunning=0 will not cause an
* incomplete IPI vmexit on the source; kvm_vcpu_check_block() handles
* this by checking vIRR one last time before blocking. The memory
* barrier implicit in set_current_state orders writing IsRunning=0
* before reading the vIRR. The processor needs a matching memory
* barrier on interrupt delivery between writing IRR and reading
* IsRunning; the lack of this barrier might be the cause of errata #1235).
*
* Clear IsRunning=0 even if guest IRQs are disabled, i.e. even if KVM
* doesn't need to detect events for scheduling purposes. The doorbell
* used to signal running vCPUs cannot be blocked, i.e. will perturb the
* CPU and cause noisy neighbor problems if the VM is sending interrupts
* to the vCPU while it's scheduled out.
*/
avic_vcpu_put(vcpu);
}