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

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Message-ID: <CALCETrWCwYMt20SD6_Os+DsudqMhX=mH1Zb9ucLsbp6gEvE93w@mail.gmail.com>
Date:	Tue, 17 Nov 2015 10:49:40 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	xen-devel <xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

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.

Also, I'd prefer X86_FEATURE_XENPV.  IMO "PV" means too many things to
too many people.

--Andy
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