linux-kernel - Re: [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to serialize

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Date:   Thu, 1 Dec 2016 17:46:08 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>, Jan Beulich <jbeulich@...e.com>
CC:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Henrique de Moraes Holschuh <hmh@....eng.br>
Subject: Re: [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to
 serialize

On 01/12/16 17:08, Andy Lutomirski wrote:
> On Thu, Dec 1, 2016 at 1:22 AM, Borislav Petkov <bp@...en8.de> wrote:
>> On Wed, Nov 30, 2016 at 12:34:55PM -0800, Andy Lutomirski wrote:
>>> Aside from being excessively slow, CPUID is problematic: Linux runs
>>> on a handful of CPUs that don't have CPUID.  MOV to CR2 is always
>>> available, so use it instead.
>>>
>>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>>> ---
>>>  arch/x86/include/asm/processor.h | 31 ++++++++-----------------------
>>>  1 file changed, 8 insertions(+), 23 deletions(-)
>> Looks nice.
>>
>> I'm wondering if we should leave this one in tip for an additional cycle
>> to have it tested on more hw. I know, it is architectural and so on but
>> who knows what every implementation actually does...
> I want the Xen opinion as well.
>
> Xen folks, can Linux use write_cr2 to serialize the CPU core on Xen PV
> or do we need something a bit heavier weight like native_write_cr2?

To sum up our conversation on IRC.

xen_write_cr2() is not serialising; it is just a write into a shared page.

native_write_cr2() would trap and be emulated.  This will incur #GP[0]
due to cpl, although not necessarily an iret on the way back out of Xen.

Something like an iret-to-self would be far quicker, and avoid trapping
into the hypervisor.

~Andrew