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Re: [PATCH v1 2/3] ARM: GICv3 ITS: do not invalidate memory while sending a command
- To: Marc Zyngier <maz@xxxxxxxxxx>
- From: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Date: Fri, 22 Sep 2023 22:02:39 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stewart Hildebrand <stewart.hildebrand@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Julien Grall <julien@xxxxxxx>
- Delivery-date: Fri, 22 Sep 2023 22:03:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH v1 2/3] ARM: GICv3 ITS: do not invalidate memory while sending a command
Hello Marc,
Marc Zyngier <maz@xxxxxxxxxx> writes:
> Volodymyr,
>
> On Fri, 22 Sep 2023 01:22:11 +0100,
> Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx> wrote:
>>
>>
>> Hi Mark,
>
> s/k/c/
Oh, I'm sorry.
>>
>> 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.
This is a great point. Thank you.
--
WBR, Volodymyr
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