linux-kernel - Re: [PATCH v4 18/30] x86/kvm/vmx: Mark vmx_l1d_should flush and vmx_l1d_flush

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Message-ID: <Z4bU2xlZXh53lgH6@google.com>
Date: Tue, 14 Jan 2025 13:19:23 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [PATCH v4 18/30] x86/kvm/vmx: Mark vmx_l1d_should flush and
 vmx_l1d_flush_cond keys as allowed in .noinstr

Please use "KVM: VMX:" for the scope.

On Tue, Jan 14, 2025, Valentin Schneider wrote:
> Later commits will cause objtool to warn about static keys being used in
> .noinstr sections in order to safely defer instruction patching IPIs
> targeted at NOHZ_FULL CPUs.
> 
> These keys are used in .noinstr code, and can be modified at runtime
> (/proc/kernel/vmx* write). However it is not expected that they will be
> flipped during latency-sensitive operations, and thus shouldn't be a source
> of interference wrt the text patching IPI.

This misses KVM's static key that's buried behind CONFIG_HYPERV=m|y.

vmlinux.o: warning: objtool: vmx_vcpu_enter_exit+0x241: __kvm_is_using_evmcs: non-RO static key usage in noinstr
vmlinux.o: warning: objtool: vmx_update_host_rsp+0x13: __kvm_is_using_evmcs: non-RO static key usage in noinstr

Side topic, it's super annoying that "objtool --noinstr" only runs on vmlinux.o.
I realize objtool doesn't have the visilibity to validate cross-object calls,
but couldn't objtool validates calls and static key/branch usage so long as the
target or key/branch is defined in the same object?