Re: [PATCH 3/5] KVM: Add a common kvm_run flag to communicate an exit needs completion
From: Chao Gao <hidden>
Date: 2025-01-13 02:09:41
Also in:
kvm, kvmarm, linux-arm-kernel, lkml
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/ (local) 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/ (local) one nit below,
quoted hunk ↗ jump to hunk
--- 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.