Re: [PATCH] KVM: selftests: Add a test for nested virtual NMI support

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Missing memory barrier before VM-exit allows compiler to reorder global variable store, causing test flakiness.
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commit 369b941892ff4fd633ee8a8bca1eef89e730d9a0
Author: Hemanth Selam <hemanth.selam@gmail.com>

KVM: selftests: Add a test for nested virtual NMI support

KVM's virtual NMI support for nested guests has no test coverage.
This commit adds a test covering the vNMI controls L1 provides in vmcb12,
including delivery of virtual NMIs to L2 and verifying the state of
V_NMI_PENDING and V_NMI_BLOCKING flags.

> diff --git a/tools/testing/selftests/kvm/x86/svm_nested_vnmi_test.c b/tools/testing/selftests/kvm/x86/svm_nested_vnmi_test.c
> new file mode 100644
> index 0000000000000..4e9916a837ddd
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/x86/svm_nested_vnmi_test.c

[ ... ]

> +static unsigned int nmi_fired;
> +
> +static void guest_nmi_handler(struct ex_regs *regs)
> +{
> +	nmi_fired++;
> +
> +	/*
> +	 * Exit to L1 from NMI context, i.e. before this handler's IRET, so
> +	 * that L1 can observe V_NMI_BLOCKING while the NMI is in service.
> +	 */
> +	vmmcall();
> +}

[Severity: Medium]
Could an optimizing compiler reorder the store to nmi_fired after the
vmmcall()?

Since nmi_fired is a plain global variable and vmmcall() is implemented as
an inline assembly block without a "memory" clobber, the C standard allows
the compiler to reorder the memory store across the vmmcall() instruction.

If instruction scheduling places the memory store after the vmmcall(), L2
will execute the VM-exit before the store is committed, causing the host (L1)
to read a stale value and making the GUEST_ASSERT_EQ(nmi_fired, i) assertion
fail sporadically.

Would adding a compiler barrier or using WRITE_ONCE() before the vmmcall()
help ensure the store is committed before exiting to the host?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260911102105.1927773-1-hemanth.selam@gmail.com?part=1