Re: [PATCH v2 4/4] KVM: selftests: Add Hyper-V hypercall forwarding test

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] KVM: selftests: missing volatile and XMM constraints in
  guest_code()
--

commit e7e0d35b7ca4b9fc386b6fe7a75c6cc9bf38431a
Author: David Woodhouse <dwmw@amazon.co.uk>

KVM: selftests: Add Hyper-V hypercall forwarding test

This commit adds a KVM selftest covering KVM_CAP_HYPERV_FORWARD_HCALL for
memory-based and fast hypercall exits as well as hypercall completion
cancellation via KVM_SET_VCPU_EVENTS.

> diff --git a/tools/testing/selftests/kvm/x86/hyperv_forward_hcall.c b/tools/testing/selftests/kvm/x86/hyperv_forward_hcall.c
> new file mode 100644
> index 0000000000000..4c290b6a10122
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/x86/hyperv_forward_hcall.c
> @@ -0,0 +1,337 @@

[ ... ]

> +static void guest_code(gpa_t pgs_gpa)
> +{
> +	__sse128_u in, out;
> +	u64 status;
> +	u8 vector;

[ ... ]

> +	/*
> +	 * Fast form: the register parameters and XMM0 reach userspace, and
> +	 * the XMM1 value userspace writes reaches the guest.
> +	 */
> +	in.as_u64[0] = XMM_IN_MARKER;
> +	in.as_u64[1] = XMM_IN_MARKER + 1;
> +	out.as_u64[0] = 0;
> +	out.as_u64[1] = 0;
> +	write_sse_reg(0, &in.vec);
> +	write_sse_reg(1, &out.vec);
> +	vector = __hyperv_hypercall(HVCALL_TEST_CODE | HV_HYPERCALL_FAST_BIT,
> +				    FAST_PARAM0, FAST_PARAM1, &status);
> +	read_sse_reg(1, &out.vec);
> +	GUEST_ASSERT(!vector);
> +	GUEST_ASSERT_EQ(status, USERSPACE_STATUS);
> +	GUEST_ASSERT_EQ(out.as_u64[0], XMM_OUT_MARKER);
> +	GUEST_ASSERT_EQ(out.as_u64[1], XMM_OUT_MARKER + 1);
> +	GUEST_SYNC(STAGE_FAST);

[Severity: Low]
Can the compiler reorder read_sse_reg() across __hyperv_hypercall(), or
clobber xmm0 before the hypercall runs?

In tools/testing/selftests/kvm/include/x86/processor.h, read_sse_reg()
uses a non-volatile asm statement with an output operand and no input
operands, and neither write_sse_reg() nor __hyperv_hypercall() declares
xmm register constraints or clobbers:

tools/testing/selftests/kvm/include/x86/processor.h:read_sse_reg() {
	switch (reg) {
	...
	case 1:
		asm("movdqa %%xmm1, %0" : "=m"(*data));
		break;
}

tools/testing/selftests/kvm/include/x86/processor.h:write_sse_reg() {
	switch (reg) {
	case 0:
		asm("movdqa %0, %%xmm0" : : "m"(*data));
		break;
}

Because x86 selftests are compiled with -O2 -march=x86-64-v2 (with SSE
enabled), the compiler can schedule a vectorized zeroing of out via xmm0
after write_sse_reg(0, &in.vec) or move read_sse_reg(1, &out.vec) ahead of
__hyperv_hypercall(), causing spurious assertion failures in
handle_fast_hcall() or guest_code().

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113170535.79246-1-dwmw2@infradead.org?part=4