Re: [Xen-devel] [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

From: Andy Lutomirski
Date: Tue Nov 17 2015 - 14:16:35 EST

On Tue, Nov 17, 2015 at 11:12 AM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 17/11/15 18:49, Andy Lutomirski wrote:
>> On Nov 17, 2015 6:40 AM, "Boris Ostrovsky" <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 11/16/2015 04:55 PM, H. Peter Anvin wrote:
>>>> On 11/16/15 12:22, Borislav Petkov wrote:
>>>>> Huh, so what's wrong with a jump:
>>>>> jmp 1f
>>>>> swapgs
>>>>> 1:
>>>> What is the point of that jump?
>>>>>> If it would make you feel better, it could be X86_BUG_XENPV :-p
>>>>> That doesn't matter - I just don't want to open the flood gates on
>>>>> pseudo feature bits.
>>>>> hpa, what do you think?
>>>> Pseudo feature bits are fine, we already have plenty of them. They make
>>>> sense as they let us reuse a lot of infrastructure.
>>> So how about something like this? And then I think we can remove usergs_sysret32 and irq_enable_sysexit pv ops completely as noone will use them (lguest doesn't set them)
>> Looks good to me. Does Xen have any sysexit/sysret32 equivalent to
>> return to 32-bit user mode? If so, it could be worth trying to wire
>> it up by patching the jz instead of the test instruction.
> From the guests point of view, there is only hypercall_iret.

Doesn't hypercall_iret have flags that ask for different behavior,
though? (VG_syscall or whatever for the 64-bit case?)

>> Also, I'd prefer X86_FEATURE_XENPV. IMO "PV" means too many things to
>> too many people.
> I agree - PV on its own is too generic.
> An alternative might be X86_FEATURE_XEN_PV_GUEST which is very clear an
> unambiguous, although rather longer.

Works for me, too, although seeing "xen_pv_host" in the Linux cpu
features would be very strange indeed :)

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