Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further

[Brian Gerst]

On Wed, Nov 6, 2019 at 4:01 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Sane ioperm() users only set the few bits in the I/O space which they need to
> access. But the update logic of the TSS I/O bitmap copies always from byte
> 0 to the last byte in the tasks bitmap which contains a zero permission bit.
>
> That means that for access only to port 65335 the full 8K bitmap has to be
> copied even if all the bytes in the TSS I/O bitmap are already filled with
> 0xff.
>
> Calculate both the position of the first zero bit and the last zero bit to
> limit the range which needs to be copied. This does not solve the problem
> when the previous tasked had only byte 0 cleared and the next one has only
> byte 65535 cleared, but trying to solve that would be too complex and
> heavyweight for the context switch path. As the ioperm() usage is very rare
> the case which is optimized is the single task/process which uses ioperm().

Here is a different idea:  We already map the TSS virtually in
cpu_entry_area.  Why not page-align the IO bitmap and remap it to the
task's bitmap on task switch?  That would avoid all copying on task
switch.

--
Brian Gerst