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Subject: Re: [PATCH 01/13] x86/entry: Fix AC assertion
To:     Brian Gerst <brgerst@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kyle Huey <me@kylehuey.com>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Robert O'Callahan <rocallahan@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>
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 <CAMzpN2i9C5Sj-M0b9Y7VtOphDJs2Z9NPux9Dg347PSeNBaXRMQ@mail.gmail.com>
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Date:   Wed, 2 Sep 2020 18:24:27 +0200
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On 02.09.20 17:58, Brian Gerst wrote:
> On Wed, Sep 2, 2020 at 9:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> From: Peter Zijlstra <peterz@infradead.org>
>>
>> The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
>> improve user entry sanity checks") unconditionally triggers on my IVB
>> machine because it does not support SMAP.
>>
>> For !SMAP hardware we patch out CLAC/STAC instructions and thus if
>> userspace sets AC, we'll still have it set after entry.
>>
>> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Acked-by: Andy Lutomirski <luto@kernel.org>
>> ---
>>   arch/x86/include/asm/entry-common.h |   11 +++++++++--
>>   1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> --- a/arch/x86/include/asm/entry-common.h
>> +++ b/arch/x86/include/asm/entry-common.h
>> @@ -18,8 +18,16 @@ static __always_inline void arch_check_u
>>                   * state, not the interrupt state as imagined by Xen.
>>                   */
>>                  unsigned long flags = native_save_fl();
>> -               WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
>> -                                     X86_EFLAGS_NT));
>> +               unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
>> +
>> +               /*
>> +                * For !SMAP hardware we patch out CLAC on entry.
>> +                */
>> +               if (boot_cpu_has(X86_FEATURE_SMAP) ||
>> +                   (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
>> +                       mask |= X86_EFLAGS_AC;
> 
> Is the explicit Xen check necessary?  IIRC the Xen hypervisor will
> filter out the SMAP bit in the cpuid pvop.

Right, and this test will nevertheless result in setting AC in the mask.
IIRC this was the main objective here.


Juergen