Re: [PATCH] KVM: selftests: Make the x86 SSE register helpers volatile

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] KVM: selftests: missing Fixes trailer for commit 676a863ce605
--

commit ec0caf7c95f47c5b258b6b71433582cb4674f3a4
Author: David Woodhouse <dwmw@amazon.co.uk>

> KVM: selftests: Make the x86 SSE register helpers volatile
>
> read_sse_reg() and write_sse_reg() were borrowed from KVM's
> _kvm_read_sse_reg() and _kvm_write_sse_reg(). The kernel is built
> without SSE, so the compiler never uses the XMM registers itself and a
> plain asm() is enough there. Selftest guest code is built with SSE, and
> there the asm statements have to tell the compiler what they do.
>
> read_sse_reg() has an output and is not volatile, so the compiler may
> treat it as a pure computation and move it, for example ahead of the
> hypercall whose XMM output it is meant to read. write_sse_reg() has no
> outputs and is therefore implicitly volatile, but it doesn't declare the
> register it overwrites, so a value the compiler was keeping there does
> not survive.
>
> Mark both volatile, and declare the register each write_sse_reg() case
> writes as clobbered. A read_sse_reg() after __hyperv_hypercall() now
> stays after it.
>
> That still leaves the compiler free to use an XMM register for its own
> code between a write_sse_reg() and the asm which consumes it.
> hyperv_write_xmm_input() in the hyperv_ipi and hyperv_tlb_flush tests
> relies on that not happening; GCC 11.5 at -O2 emits nothing there.
> Closing that window entirely would mean passing the XMM values as
> operands of the hypercall asm itself.
>
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/all/sashiko-outbox-209724@kernel.org/
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

[Severity: Low]
Since this fixes read_sse_reg() and write_sse_reg(), should it include:

  Fixes: 676a863ce605 ("KVM: selftests: Better XMM read/write helpers")

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113181053.81779-1-dwmw2@infradead.org?part=1