Re: [PATCH for-next 07/16] xen/arm: Introduce copy_to_guest_phys_flush_dcache

[Julien Grall <julien.grall@linaro.org>]

Hi Andrew,

On 23/11/17 18:49, Andrew Cooper wrote:
> On 23/11/17 18:32, Julien Grall wrote:
>> This new function will be used in a follow-up patch to copy data to the guest
>> using the IPA (aka guest physical address) and then clean the cache.
>>
>> Signed-off-by: Julien Grall <julien.grall@linaro.org>
>> ---
>>   xen/arch/arm/guestcopy.c           | 10 ++++++++++
>>   xen/include/asm-arm/guest_access.h |  6 ++++++
>>   2 files changed, 16 insertions(+)
>>
>> diff --git a/xen/arch/arm/guestcopy.c b/xen/arch/arm/guestcopy.c
>> index be53bee559..7958663970 100644
>> --- a/xen/arch/arm/guestcopy.c
>> +++ b/xen/arch/arm/guestcopy.c
>> @@ -110,6 +110,16 @@ unsigned long raw_copy_from_guest(void *to, const void __user *from, unsigned le
>>                         COPY_from_guest | COPY_linear);
>>   }
>>   
>> +unsigned long copy_to_guest_phys_flush_dcache(struct domain *d,
>> +                                              paddr_t gpa,
>> +                                              void *buf,
>> +                                              unsigned int len)
>> +{
>> +    /* P2M is shared between all vCPUs, so the vCPU used does not matter. */
> 
> Be very careful with this line of thinking.  It is only works after
> DOMCTL_max_vcpus has succeeded, and before that point, it is a latent
> NULL pointer dereference.

I really don't expect that function been used before DOMCT_max_vcpus is 
set. It is only used for hardware emulation or Xen loading image into 
the hardware domain memory. I could add a check d->vcpus to be safe.

> 
> Also, what about vcpus configured with alternative views?

It is not important because the underlying call is get_page_from_gfn 
that does not care about the alternative view (that function take a 
domain in parameter). I can update the comment.

Cheers,

-- 
Julien Grall

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