Re: [PATCH] x86/traps: Avoid the #GP slowpath for guest #DB exceptions

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/01/15 14:00, Jan Beulich wrote:
>>>> On 08.01.15 at 14:25, <andrew.cooper3@citrix.com> wrote:
>> do_debug() is capable of correctly dealing with #DB exceptions in guest
>> context, and indeed needs to be as the 'icebp' instruction skips the DPL
>> check anyway.
> I don't follow: ICEBP doesn't check DPL, right, but what does setting
> DPL to 3 buy us? Other than for INTO and INT3, we don't want to
> encourage use of INT $0x01 instructions, nor am I aware of anyone
> commonly using them. Yet afaict only they would be affected by your
> change. Actual #DB, just like any other hardware exceptions, don't
> consider DPL just like ICEBP doesn't.
>
> Jan
>

I noticed it as I was attempting to get my debugtraps test case working
for PV guests, which does indeed use 'int $0x1'.  But as you point out,
it is only 'int $0x1' we would gain a fastpath for, which doesn't really
make it worthwhile.

~Andrew