linux-kernel - Re: [PATCH 1/6] x86/entry: Introduce local_db

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Message-ID: <8831da5e-b65d-a4e8-2a3b-ac46282f4591@citrix.com>
Date:   Thu, 28 May 2020 22:36:54 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, luto@...capital.net,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        sean.j.christopherson@...el.com, daniel.thompson@...aro.org
Subject: Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}()

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