Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage

Paolo Bonzini
2021-11-06T07:06:01Z

On 11/5/21 19:17, Sean Christopherson wrote:
> On Thu, Nov 04, 2021, Peter Zijlstra wrote:
>> In the vmread exceptin path, use the, thus far, unused output register
>> to push the @fault argument onto the stack. This, in turn, enables the
>> exception handler to not do pushes and only modify that register when
>> an exception does occur.
>>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> ---
>> arch/x86/kvm/vmx/vmx_ops.h | 14 ++++++--------
>> 1 file changed, 6 insertions(+), 8 deletions(-)
>>
>> --- a/arch/x86/kvm/vmx/vmx_ops.h
>> +++ b/arch/x86/kvm/vmx/vmx_ops.h
>> @@ -80,9 +80,11 @@ static __always_inline unsigned long __v
>> * @field, and bounce through the trampoline to preserve
>> * volatile registers.
>> */
>> - "push $0\n\t"
>> + "xorl %k1, %k1\n\t"
>> + "2:\n\t"
>> + "push %1\n\t"
>> "push %2\n\t"
>
> This trick doesn't work if the compiler selects the same GPR for %1 and %2, as
> the "field" will get lost.

Ouch, good catch. It should be actually very simple to fix it, just
mark "value" as an "early clobber" output:

: ASM_CALL_CONSTRAINT, "=&r"(value) : "r"(field) : "cc");

That's an output which is written before the instruction is finished
using the input operands. The manual even says "this operand may not
lie in a register that is read by the instruction or as part of any
memory address", which is exactly what you caught with %1 and %2 both
being the same GPR.

Paolo