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

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.


> Signed-off-by: Oleksandr Dmytryshyn <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx>
> ---
>  xen/arch/arm/guestcopy.c | 2 ++
>  1 file changed, 2 insertions(+)
> diff --git a/xen/arch/arm/guestcopy.c b/xen/arch/arm/guestcopy.c
> index d146cd6..28d3151 100644
> --- a/xen/arch/arm/guestcopy.c
> +++ b/xen/arch/arm/guestcopy.c
> @@ -24,6 +24,7 @@ unsigned long raw_copy_to_guest(void *to, const void *from, 
> unsigned len)
>          p = map_domain_page(g>>PAGE_SHIFT);
>          p += offset;
>          memcpy(p, from, size);
> +        flush_xen_dcache_va_range(p, size);
>          unmap_domain_page(p - offset);
>          len -= size;
> @@ -54,6 +55,7 @@ unsigned long raw_clear_guest(void *to, unsigned len)
>          p = map_domain_page(g>>PAGE_SHIFT);
>          p += offset;
>          memset(p, 0x00, size);
> +        flush_xen_dcache_va_range(p, size);
>          unmap_domain_page(p - offset);
>          len -= size;

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