Re: [PATCH 06/24] x86,kvm: Fix indirect calls in emulator

[Paolo Bonzini]

On 24/01/2018 11:35, Peter Zijlstra wrote:
> On Tue, Jan 23, 2018 at 08:48:13PM +0000, David Woodhouse wrote:
>> On Tue, 2018-01-23 at 21:28 +0100, Borislav Petkov wrote:
>>>
>>>> ÂÂÂÂÂÂÂflags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
>>>> -ÂÂÂÂÂasm("push %[flags]; popf; call *%[fastop]"
>>>> -ÂÂÂÂÂÂÂÂ : "=a"(rc) : [fastop]"r"(fop), [flags]"r"(flags));
>>>> +ÂÂÂÂÂasm("push %[flags]; popf; " CALL_NOSPEC
>>>> +ÂÂÂÂÂÂÂÂ : "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));
>>>
>>> Oh, "thunk_target" is magical.
>>
>> You can use THUNK_TARGET(fop), which will be "rm" on 32-bit and avoids
>> register starvation in some cases (I don't think the hyperv calls
>> worked until I did that).
> 
> The reason I didn't use THUNK_TARGET() was exactly because it used "rm"
> and the current code did "r" only. I'm happy to change if people can
> agree on something ;-)

In practice, "fastop" is going to be in a register because of how it's
computed, but "rm" is okay.

Thanks,

Paolo