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Subject: Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, luto@amacapital.net,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        sean.j.christopherson@intel.com, daniel.thompson@linaro.org
References: <20200528201937.038455891@infradead.org>
 <20200528202328.418625592@infradead.org>
 <17e097f5-f92c-bd7e-fb1d-ee08c4169dbe@citrix.com>
 <20200528211550.GR2483@worktop.programming.kicks-ass.net>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Message-ID: <8831da5e-b65d-a4e8-2a3b-ac46282f4591@citrix.com>
Date:   Thu, 28 May 2020 22:36:54 +0100
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On 28/05/2020 22:15, Peter Zijlstra wrote:
> On Thu, May 28, 2020 at 09:52:30PM +0100, Andrew Cooper wrote:
>> On 28/05/2020 21:19, Peter Zijlstra wrote:
>>> --- a/arch/x86/include/asm/debugreg.h
>>> +++ b/arch/x86/include/asm/debugreg.h
>>> @@ -113,6 +113,31 @@ static inline void debug_stack_usage_inc
>>>  static inline void debug_stack_usage_dec(void) { }
>>>  #endif /* X86_64 */
>>>  
>>> +static __always_inline void local_db_save(unsigned long *dr7)
>>> +{
>>> +	get_debugreg(*dr7, 7);
>>> +	if (*dr7)
>>> +		set_debugreg(0, 7);
>> %dr7 has an architecturally stuck bit in it.
>>
>> You want *dr7 != 0x400 to avoid writing 0 unconditionally.
> Do we have to have that bit set when writing it? Otherwise I might
> actually prefer masking it out.

Not currently.  I guess it depends on how likely %dr7 is to gain an
inverted polarity bit like %dr6 did.

~Andrew