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Re: [Xen-devel] [PATCH 04/11] x86: Fix calculation of %dr6/7 reserved bits
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 6 Jun 2018 15:52:22 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 06 Jun 2018 14:52:58 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 06/06/18 15:50, Andrew Cooper wrote:
> On 06/06/18 15:16, Jan Beulich wrote:
>>> + dr6 |= 0xfffe0ff0 | (rtm ? 0 : X86_DR6_RTM);
>>> + dr6 &= 0xffffefff;
>> I'm not overly happy with this move to literal numbers. Could we
>> at least meet in the middle and adjust the first line to
>>
>> dr6 |= X86_DR6_DEFAULT & ~(rtm ? 0 : X86_DR6_RTM);
>>
>> ? Even the second line, it doesn't look unreasonable to me to
>> accompany the other X86_DR6_* values you introduce with
>> X86_DR6_MBZ (or some such, if you dislike this name). Of course
>> then for the first line using X86_DR6_MBS would also be an option,
>> allowing you to retain the | (which documentation-wise might be
>> slightly better than the & ~()).
> At the moment, every single use of DR_*_RESERVED_* in the entire
> codebase is buggy, although this is admittedly a side effect of the
> introduction of RTM (and that the advice given the Intel manual has
> caused software not to be forwards compatible to the introduction of
> RTM. I've also arranged for that advice to be fixed.)
>
> Irrespective, the constants are now simply appropriate to express how
Apologies - I meant "simply not appropriate".
~Andrew
> the reserved bit behaviour works, which is why I'm removing them and
> introducing these functions instead.
>
> As for naked numbers, they are like this because its the clearest I
> could make the code. Hiding these behind defines makes it harder to
> cross reference with the comment.
>
> An alternative might be to go for an ARM approach using GENMASK()/BIT(),
> but I'm not a fan of those because it introduces more confusion as to
> where the boundaries of the mask lie.
>
> Like you, I'm not a fan of naked numbers, but in this instance, I chose
> them because I think they are the least bad option available.
>
> ~Andrew
>
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