Re: [Xen-devel] [PATCH 2/5] xen/arm: optee: check for preemption while freeing shared buffers

[Julien Grall <julien.grall@arm.com>]

Hi Volodymyr,

On 9/11/19 7:53 PM, Volodymyr Babchuk wrote:
> 
> Julien Grall writes:
> 
>> Hi Volodymyr,
>>
>> On 8/23/19 7:48 PM, Volodymyr Babchuk wrote:
>>> Now we have limit for one shared buffer size, so we can be sure that
>>> one call to free_optee_shm_buf() will not free all
>>> MAX_TOTAL_SMH_BUF_PG pages at once. Thus, we now can check for
>>> hypercall_preempt_check() in the loop inside
>>> optee_relinquish_resources() and this will ensure that we are not
>>> missing preemption.
>>
>> I am not sure to understand the correlation between the two
>> sentences. Even if previously the guest could pin up to
>> MAX_TOTAL_SHM_BUF_PG in one call, a well-behaved guest would result to
>> do multiple calls and therefore preemption would have been useful.
> Looks like now I don't understand you.
> 
> I'm talking about shared buffers. We have limited shared buffer to some
> reasonable size. There is bad- or well-behaving guests in this context,
> because guest can't share one big buffer in multiple calls. In other
> worlds, if guest *needs* to share 512MB buffer with OP-TEE, it will be
> forced to do this in one call. But we are forbidding big buffers right
> now.
> 
> optee_relinquish_resources() is called during domain destruction. At
> this time we can have a number of still living shared buffers, each of
> one is no bigger than 512 pages. Thanks to this, we can call
> hypercall_preempt_check() only in optee_relinquish_resources(), but not
> in free_optee_shm_buf().

I understand what you mean, however my point is that this patch does not 
dependent of the previous patch. Even if this patch goes alone, you will 
improve well-behaved guest. For ill-behaved guest, the problem will stay 
the same so no change.

> 
> If we will allow guest to register bigger buffer, than we will be forced
> to check for preemption in free_optee_shm_buf() as well.

Well yes, however this patch would still be useful independently of the 
size of the buffer.

Cheers,

-- 
Julien Grall

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