Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel]

On 6 August 2018 at 13:19, Siddhesh Poyarekar <siddhesh@xxxxxxxxxx> wrote:
> On 08/06/2018 04:01 PM, Mikulas Patocka wrote:
>>
>> I think there are three possible solutions:
>>
>> 1. provide an alternative memcpy implementation that doesn't do unaligned
>> accesses and recompile the graphics software with -mstrict-align
>
>
> Given that there's already a tunable glibc.cpu.cached_memopt for powerpc
> that (as Tulio clarified elsewhere) essentially does the same thing for
> cache-inhibited memory, it wouldn't be too much of an overhead to put in
> another ifunc implementation that gets chosen only when one sets this
> tunable.  In fact, we could reuse the C string routines for this to avoid
> adding yet another assembly implementation to have to support. That way we
> can minimally fix the issue at hand without regressing existing uses.
>
> You can then set the glibc.cpu.cached_memopt tunable in the default
> environment for your board[1] or for applications that need it (e.g.
> whenever DISPLAY is exported or something like that).
>
> The only difference from Power would be that cpu.noncached==0 for Power by
> default whereas for aarch64 it will be the other way around.  It shouldn't
> be too hard to enhance the framework to set platform-specific defaults.
>

Thanks Siddhesh,

But we don't need another memcpy(). We need outbound PCIe windows that
tolerate being mapped as normal non-cacheable memory.

And if this is fundamentally impossible, can someone please try
explaining it again? (apologies for being thick)