linux-kernel - Re: [PATCH 3/5] KVM: Add a common kvm_run flag to communicate an exit needs completion

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Message-ID: <Z4R12HOD1o8ETYzm@intel.com>
Date: Mon, 13 Jan 2025 10:09:28 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
	"Oliver Upton" <oliver.upton@...ux.dev>, Michael Ellerman
	<mpe@...erman.id.au>, <kvm@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] KVM: Add a common kvm_run flag to communicate an
 exit needs completion

On Fri, Jan 10, 2025 at 05:24:48PM -0800, Sean Christopherson wrote:
>Add a kvm_run flag, KVM_RUN_NEEDS_COMPLETION, to communicate to userspace
>that KVM_RUN needs to be re-executed prior to save/restore in order to
>complete the instruction/operation that triggered the userspace exit.
>
>KVM's current approach of adding notes in the Documentation is beyond
>brittle, e.g. there is at least one known case where a KVM developer added
>a new userspace exit type, and then that same developer forgot to handle
>completion when adding userspace support.

This answers one question I had:
https://lore.kernel.org/kvm/Z1bmUCEdoZ87wIMn@intel.com/

So, it is the VMM's (i.e., QEMU's) responsibility to re-execute KVM_RUN in this
case.

Btw, can this flag be used to address the issue [*] with steal time accounting?
We can set the new flag for each vCPU in the PM notifier and we need to change
the re-execution to handle steal time accounting (not just IO completion).

[*]: https://lore.kernel.org/kvm/Z36XJl1OAahVkxhl@google.com/

one nit below,

>--- a/arch/x86/include/uapi/asm/kvm.h
>+++ b/arch/x86/include/uapi/asm/kvm.h
>@@ -104,9 +104,10 @@ struct kvm_ioapic_state {
> #define KVM_IRQCHIP_IOAPIC       2
> #define KVM_NR_IRQCHIPS          3
> 
>-#define KVM_RUN_X86_SMM		 (1 << 0)
>-#define KVM_RUN_X86_BUS_LOCK     (1 << 1)
>-#define KVM_RUN_X86_GUEST_MODE   (1 << 2)
>+#define KVM_RUN_X86_SMM			(1 << 0)
>+#define KVM_RUN_X86_BUS_LOCK		(1 << 1)
>+#define KVM_RUN_X86_GUEST_MODE		(1 << 2)
>+#define KVM_RUN_X86_NEEDS_COMPLETION	(1 << 2)

This X86_NEEDS_COMPLETION should be dropped. It is never used.