linux-kernel - Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync

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Message-ID: <168ef9ef-a2dc-1c7f-2462-b1ce0cc40c71@citrix.com>
Date:   Fri, 2 Dec 2016 17:26:26 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Xen-devel List <xen-devel@...ts.xen.org>,
        Juergen Gross <JGross@...e.com>,
        Borislav Petkov <bp@...en8.de>,
        Matthew Whitehead <tedheadster@...il.com>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Brian Gerst <brgerst@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()
 implementation

On 02/12/16 17:23, Andy Lutomirski wrote:
> On Fri, Dec 2, 2016 at 9:16 AM, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>> On 02/12/16 17:07, Andy Lutomirski wrote:
>>> On Dec 2, 2016 3:44 AM, "Andrew Cooper" <andrew.cooper3@...rix.com> wrote:
>>>> On 02/12/16 00:35, Andy Lutomirski wrote:
>>>>> On Xen PV, CPUID is likely to trap, and Xen hypercalls aren't
>>>>> guaranteed to serialize.  (Even CPUID isn't *really* guaranteed to
>>>>> serialize on Xen PV, but, in practice, any trap it generates will
>>>>> serialize.)
>>>> Well, Xen will enabled CPUID Faulting wherever it can, which is
>>>> realistically all IvyBridge hardware and newer.
>>>>
>>>> All hypercalls are a privilege change to cpl0.  I'd hope this condition
>>>> is serialising, but I can't actually find any documentation proving or
>>>> disproving this.
>>> I don't know for sure.  IRET is serializing, and if Xen returns using
>>> IRET, we're fine.
>> All returns to a 64bit PV guest at defined points (hypercall return,
>> exception entry, etc) are from SYSRET, not IRET.
> But CPUID faulting isn't like this, right?  Unless Xen does
> opportunistic SYSRET.

Correct.  Xen doesn't do opportunistic SYSRET.

>
>> Talking of, I still have a patch to remove
>> PARAVIRT_ADJUST_EXCEPTION_FRAME which I need to complete and send upstream.
>>
>>>>> On my laptop, CPUID(eax=1, ecx=0) is ~83ns and IRET-to-self is
>>>>> ~110ns.  But Xen PV will trap CPUID if possible, so IRET-to-self
>>>>> should end up being a nice speedup.
>>>>>
>>>>> Cc: Andrew Cooper <andrew.cooper3@...rix.com>
>>>>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>>>> CC'ing xen-devel and the Xen maintainers in Linux.
>>>>
>>>> As this is the only email from this series in my inbox, I will say this
>>>> here, but it should really be against patch 6.
>>>>
>>>> A write to %cr2 is apparently (http://sandpile.org/x86/coherent.htm) not
>>>> serialising on the 486, but I don't have a manual to hand to check.
>>> I'll quote the (modern) SDM.  For self-modifying code "The use of one
>>> of these options is not required for programs intended to run on the
>>> Pentium or Intel486 processors,
>>> but are recommended to ensure compatibility with the P6 and more
>>> recent processor families.".  For cross-modifying code "The use of
>>> this option is not required for programs intended to run on the
>>> Intel486 processor, but is recommended
>>> to ensure compatibility with the Pentium 4, Intel Xeon, P6 family, and
>>> Pentium processors."  So I'm not sure there's a problem.
>> Fair enough.  (Assuming similar properties hold for the older processors
>> of other vendors.)
> No, you were right -- a different section of the SDM contradicts it:
>
> For Intel486 processors, a write to an instruction in the cache will
> modify it in both the cache and memory, but if
> the instruction was prefetched before the write, the old version of
> the instruction could be the one executed. To
> prevent the old instruction from being executed, flush the instruction
> prefetch unit by coding a jump instruction
> immediately after any write that modifies an instruction.

:(

Presumably this means patching has been subtly broken forever on the 486?

~Andrew