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SubjectRe: [PATCH v2 3/5] KVM: VMX: Add error handling to VMREAD helper
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On 28/07/19 21:36, Josh Poimboeuf wrote:
> On Fri, Jul 19, 2019 at 01:41:08PM -0700, Sean Christopherson wrote:
>> @@ -68,8 +67,22 @@ static __always_inline unsigned long __vmcs_readl(unsigned long field)
>> {
>> unsigned long value;
>>
>> - asm volatile (__ex_clear("vmread %1, %0", "%k0")
>> - : "=r"(value) : "r"(field));
>> + asm volatile("1: vmread %2, %1\n\t"
>> + ".byte 0x3e\n\t" /* branch taken hint */
>> + "ja 3f\n\t"
>> + "mov %2, %%" _ASM_ARG1 "\n\t"
>> + "xor %%" _ASM_ARG2 ", %%" _ASM_ARG2 "\n\t"
>> + "2: call vmread_error\n\t"
>> + "xor %k1, %k1\n\t"
>> + "3:\n\t"
>> +
>> + ".pushsection .fixup, \"ax\"\n\t"
>> + "4: mov %2, %%" _ASM_ARG1 "\n\t"
>> + "mov $1, %%" _ASM_ARG2 "\n\t"
>> + "jmp 2b\n\t"
>> + ".popsection\n\t"
>> + _ASM_EXTABLE(1b, 4b)
>> + : ASM_CALL_CONSTRAINT, "=r"(value) : "r"(field) : "cc");
>
> Was there a reason you didn't do the asm goto thing here like you did
> for the previous patch? That seemed cleaner, and needs less asm.

It's because asm goto doesn't support outputs.

Paolo

> I think the branch hints aren't needed -- they're ignored on modern
> processors. Ditto for the previous patch.
>
> Also please use named asm operands whereever you can, like "%[field]"
> instead of "%2". It helps a lot with readability.
>

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