Re: [PATCH 4/4] KVM: selftests: Check VMPTRLD with active eVMCS

[Vitaly Kuznetsov <vkuznets@redhat.com>]

sashiko-bot@kernel.org writes:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] The hardcoded `VMPTRLD_INSN_SIZE` of 5 bytes is brittle
> because `vmptrld` is executed using an inline assembly memory operand
> constraint (`"m"`), which results in a variable-sized instruction
> depending on compiler optimizations and register allocation.

True that, but afair we don't currently have an instruction decoder in
selftests. I see three ways to move forward:

- Calculate the size of the instruction with two labels, something like
  (untested):

    static inline int __vmptrld(u64 vmcs_pa, int *insn_size)
    {
            u8 ret;

            __asm__ __volatile__ ("movl $(2f-1f), %[size];"
                                  "1: vmptrld %[pa]; 2:"
                                  "setna %[ret]"
                    : [ret]"=rm"(ret), [size]"=m"(*insn_size)
                    : [pa]"m"(vmcs_pa)
                    : "cc", "memory");

            return ret;
    }

  and use insn_size in the tests.

- Stuf __vmptrld() with NOPs after vmptrld and keep the constant jump,
  just make it loner (e.g. 10 bytes should realistically be enough).

- Make peace with the fragility (it's just a selftest after all) and
  leave a comment in vmptrld() that changing the asm there make break
  the assumption.

Any preferences? Personally, I'm feeling adventurous and can go with the
first one - calculating the exact size with labels.

-- 
Vitaly