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

[Paolo Bonzini]

On 25/01/2018 10:34, Peter Zijlstra wrote:
> On Wed, Jan 24, 2018 at 11:43:05AM +0100, Paolo Bonzini wrote:
>> 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.
> 
> OK, so the other occurence in that file uses "+S", which is the SI
> register. That cannot use THUNK_TARGET(), right?

Nope, it reads the output in %esi too.

> So do you want one THUNK_TARGET and one open coded, or keep the patch as
> is (both open coded) ?

Open coded is okay.

Paolo