Re: [Qemu-devel] [PATCH 0/2] coroutine: add x86 specific coroutine backend

[Paolo Bonzini]

On 13/03/19 12:49, Peter Maydell wrote:
> On Mon, 11 Mar 2019 at 12:49, Paolo Bonzini <address@hidden> wrote:
>>
>> This is in preparation for CET support for x86.
> 
> What's CET ?

It's basically shadow stacks and tracking of indirect branch
targets---hardware support for what others calls control-flow integrity.
 I've now sent the full series, which consists of these patches (with a
few cleanups) followed by CET support.

>> I know this is very late, but if possible I'd like to have this in 4.0
>> as an experimental backend.
>>
>> Paolo Bonzini (3):
>>   qemugdb: fix licensing
>>   qemugdb: allow adding support for other coroutine backends
>>   coroutine: add x86 specific coroutine backend
> 
> I can't say I'm terribly enthusiastic about having
> native-asm specific versions of the coroutine code.
> It seems like the kind of thing that's going to be
> fragile against changes in libc implementation, calling
> convention, etc.

I thought the same before I started writing it, but surprisingly it does
not require any of that.  Since the asm clobbers all registers except
the frame and stack pointer, the compiler will just push/pop all
caller-save registers automatically in qemu_coroutine_switch and
qemu_coroutine_new's prolog and epilog.  This also means that porting it
to other architectures should require only a rewrite of the CO_SWITCH macro.

I can try porting it to ARM and/or aarch64 to prove that point, or
perhaps David will enjoy beating me and doing an s390 port. :)

Paolo