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

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Date:	Tue, 17 Nov 2015 19:12:43 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Borislav Petkov <bp@...en8.de>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

On 17/11/15 18:49, Andy Lutomirski wrote:
> On Nov 17, 2015 6:40 AM, "Boris Ostrovsky" <boris.ostrovsky@...cle.com> 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.

>
> 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.

~Andrew
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