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From: Andy Lutomirski <luto@amacapital.net>
Date: Tue, 17 Nov 2015 10:49:40 -0800
Message-ID: <CALCETrWCwYMt20SD6_Os+DsudqMhX=mH1Zb9ucLsbp6gEvE93w@mail.gmail.com>
Subject: Re: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: xen-devel <xen-devel@lists.xen.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        David Vrabel <david.vrabel@citrix.com>, Borislav Petkov <bp@alien8.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
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On Nov 17, 2015 6:40 AM, "Boris Ostrovsky" <boris.ostrovsky@oracle.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