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Re: [Xen-devel] [PATCH] xen: arm: add missing flushing dcache to the copy to/clean guest functions

Hi to all.

Currently we use Dom0 kernel size about 30 MBytes (on Panda5 board).

I've measured a flushing time in the 2 cases:
1. Flush about 30 MBytes (flush_xen_dcache_va_range() function) -  1280 uS
2. Full dcache flush (I've ported this from u-boot to the hypervisor) - 93 uS

I've used OMAP5 ES2.0 for test (Panda5 board). I've used 32K_TIMER in OMAP5
to measure the flushing time.

The problem exists only for the device tree image (for Dom0), because the kernel
Dom0 image (and ramdisk image) is copied with flushing the dcache.

Oleksandr Dmytryshyn | Product Engineering and Development
P x3657  M +38.067.382.2525


On Mon, Nov 25, 2013 at 7:06 PM, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
> On 11/25/2013 04:33 PM, Ian Campbell wrote:
>> On Mon, 2013-11-25 at 18:21 +0200, Oleksandr Dmytryshyn wrote:
>> Thanks for the patch.
>>> Without flushing dcache the hypervisor couldn't copy the device tree
>>> correctly when booting the kernel dom0 Image (memory with device tree
>>> is corrupted). As the result - when we try to load the kernel dom0
>>> Image - dom0 hungs frequently. This issue is not reproduced with the
>>> kernel dom0 zImage because the zImage decompressor code flushes all
>>> dcache before starting the decompressed kernel Image. When the
>>> hypervisor loads the kernel uImage or initrd, this memory region
>>> isn't corrupted because the hypervisor code flushes the dcache.
>> So if not then when/how is this reproduced?
>> In general I would like to try and keep flushes out of this code path
>> because they are used in the hypercall path, we have decreed that
>> guest's must have caching enabled to make hypercalls (at least those
>> which take in-memory arguments).
>> I think the right fix is to do the flushes domain_build.c, similar to
>> how kernel_zimage_load does it. This might need an opencoded version of
>> copy_to_user. Or better, introduce a flushing variant which shares most
>> code with the normal one via a common internal function.
>> Or perhaps we should flush all of the new guest's RAM after building. I
>> think Julien was looking at doing something along those lines for the
>> domU building case.
> I was planning to handle only domU, but the issue can also happen with
> dom0 (which is finally a guest as another :)).
> The main reason behind the both issues is because Linux is starting with
> cache disabled. So if Xen/Dom0 are copying data in a guest, some data
> can stay in the cache for a while. In this case the guest will see
> corrupted data.
> Flushing all the new RAM seems a bit complicated and slow (AFAIK, you
> can only flush data cache page by page). I would stay on a similar
> solution and check if the guest is already running or not.
> Ian, what do you think?
> --
> Julien Grall

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