Re: [Xen-devel] [PATCH] x86/HVM: fix hvmemul_rep_outs_set_context()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/11/17 15:09, Jan Beulich wrote:
> There were two issues with this function: Its use of
> hvmemul_do_pio_buffer() was wrong (the function deals only with
> individual port accesses, not repeated ones, i.e. passing it
> "*reps * bytes_per_rep" does not have the intended effect). And it
> could have processed a larger set of operations in one go than was
> probably intended (limited just by the size that xmalloc() can hand
> back).
>
> By converting to proper use of hvmemul_do_pio_buffer(), no intermediate
> buffer is needed at all. As a result a preemption check is being added.
>
> Also drop unused parameters from the function.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

While this does look like real bug, and bugfix, it isn't the issue I'm
hitting.  I've distilled the repro scenario down to a tiny XTF test,
which is just a `rep outsb` with a buffer which crosses a page boundary.

The results are reliably:

(d1) --- Xen Test Framework ---
(d1) Environment: HVM 32bit (No paging)
(d1) Test hvm-print
(d1) String crossing a page boundary
(XEN) MMIO emulation failed (1): d1v0 32bit @ 0010:001032b0 -> 5e c3 8d
b4 26 00 00 00 00 8d bc 27 00 00 00 00
(d1) Test result: SUCCESS

The Port IO hits a retry because of hitting the page boundary, and the
retry logic successes, as evident by all data hitting hvm_print_line(). 
Somewhere however, the PIO turns into MMIO, and a failure is reported
after the PIO completed successfully.  %rip in the failure message
points after the `rep outsb`, rather than at it.

If anyone has any ideas, I'm all ears.  If not, I will try to find some
time to look deeper into the issue.

~Andrew

Attachment: 0001-MMIO-failure-trigger.patch
Description: Text Data

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