Re: [PATCH v1 2/3] ARM: GICv3 ITS: do not invalidate memory while sending a command

[Marc Zyngier <maz@xxxxxxxxxx>]

Volodymyr,

On Fri, 22 Sep 2023 01:22:11 +0100,
Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx> wrote:
> 
> 
> Hi Mark,

s/k/c/

> 
> I am writing to you, because you are GICv3 maintainer in Linux. We are
> updating ITS driver in Xen and we have a question about cache
> maintenance WRT memory shared with ITS. As I can see, the Linux ITS
> driver uses gic_flush_dcache_to_poc() all over the code. This boils down
> to "dc civac" instruction which does both clean and invalidate. But do
> we really need to invalidate a cache when we are sending an ITS command?
> In my understanding it is sufficient to clean a cache only and Linux
> uses clean&invalidate just out of convenience. Is this correct?

It really depends how you look at it. We use DC CIVA as the standard
way to give a buffer to a device, as that's what the DMA API
does. Switching to a simple clean is possible, but I don't really see
what it brings you.

ITS commands are usually written as a single command followed by a
SYNC/VSYNC. That's a total a 8 64bit words, which makes a cache line
on 99.999% of the implementations.

What do you gain by keeping the cache line around? Not much. By the
time you go around the command queue and need the same data again, it
will have been evicted from your L1 already.

So while I don't see a problem with what you are suggesting, I also
think the change is pretty much irrelevant.

HTH,

        M.

-- 
Without deviation from the norm, progress is not possible.