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

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Message-ID: <CALCETrXv2=JUJvcx0_wmd8bQQ8UUAOyhX+_LTKch8RaJFspPcw@mail.gmail.com>
Date:	Tue, 17 Nov 2015 11:16:11 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"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 Tue, Nov 17, 2015 at 11:12 AM, Andrew Cooper
<andrew.cooper3@...rix.com> wrote:
> 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.

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 :)

--Andy
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