RE: [PATCH v4 1/4] x86/cpufeatures: Enumerate #DB for bus lock detection

[Thomas Gleixner]

Fenghua,

On Wed, Jan 27 2021 at 22:39, Fenghua Yu wrote:
>> On Wed, Jan 27, 2021 2:16 PM, Thomas Gleixner wrote:
>> On Tue, Nov 24 2020 at 20:52, Fenghua Yu wrote:
>> 
>> > A bus lock is acquired though either split locked access to writeback
>> > (WB) memory or any locked access to non-WB memory. This is typically
>> > >1000 cycles slower than an atomic operation within a cache line. It
>> > also disrupts performance on other cores.
>> >
>> > Some CPUs have ability to notify the kernel by an #DB trap after a
>> > user instruction acquires a bus lock and is executed. This allows the
>> > kernel to enforce user application throttling or mitigations.
>> 
>> That's nice, but how does that interact with a data breakpoint on the same
>> location?
>
> If both data breakpoint and bus lock happen on the same location, the bus lock
> is handled first and then the data breakpoint is handled in the same exception:
>
> 1. If warn on bus lock, a rate limited warning is printed for the bus lock and then
>     a SIGTRAP is sent to the user process.
> 2. If fatal on bus lock, a SIGBUS is sent to the user process for the bus lock and a
>     SIGTRAP is also sent to the user process. I think the SIGBUS will be delivered first
>     to the process and then SIGTRAP will be delivered to the process.
> 3. If ratelimit on bus lock, first the tasks in the user sleep for specified time, then
>     SIGTRAP is sent to the user process.
>
> Is the interaction OK?

The ordering is a software choice and fine with me as long as the
hardware actually delivers both.

All of this information needs to be in the changelog of the relevant
patches.

Thanks,

        tglx