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 09:08:01 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Borislav Petkov <bp@...en8.de>, Jan Beulich <jbeulich@...e.com>,
        Andrew Cooper <andrew.cooper3@...rix.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 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?

--Andy