From: Julien Grall <julien.grall@arm.com>
To: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Cc: "tee-dev@lists.linaro.org" <tee-dev@lists.linaro.org>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/5] xen/arm: optee: check for preemption while freeing shared buffers
Date: Thu, 12 Sep 2019 20:39:18 +0100 [thread overview]
Message-ID: <2a5adc32-80fb-d67f-46e5-074ca92fa372@arm.com> (raw)
In-Reply-To: <87impyfyw6.fsf@epam.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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next prev parent reply other threads:[~2019-09-12 19:39 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-23 18:48 [Xen-devel] [PATCH 0/5] arch/arm: optee: fix TODOs and remove "experimental" status Volodymyr Babchuk
2019-08-23 18:48 ` [Xen-devel] [PATCH 1/5] xen/arm: optee: impose limit on shared buffer size Volodymyr Babchuk
2019-09-09 22:11 ` Julien Grall
2019-09-11 18:48 ` Volodymyr Babchuk
2019-09-12 19:32 ` Julien Grall
2019-09-12 19:45 ` Volodymyr Babchuk
2019-09-12 19:51 ` Julien Grall
2019-09-16 15:26 ` Volodymyr Babchuk
2019-09-17 10:49 ` Julien Grall
2019-09-17 12:28 ` Volodymyr Babchuk
2019-09-17 18:46 ` Julien Grall
2019-08-23 18:48 ` [Xen-devel] [PATCH 2/5] xen/arm: optee: check for preemption while freeing shared buffers Volodymyr Babchuk
2019-09-09 22:19 ` Julien Grall
2019-09-11 18:53 ` Volodymyr Babchuk
2019-09-12 19:39 ` Julien Grall [this message]
2019-09-12 19:47 ` Volodymyr Babchuk
2019-08-23 18:48 ` [Xen-devel] [PATCH 3/5] xen/arm: optee: limit number of " Volodymyr Babchuk
2019-08-23 18:48 ` [Xen-devel] [PATCH 4/5] xen/arm: optee: handle share buffer translation error Volodymyr Babchuk
2019-09-10 11:17 ` Julien Grall
2019-09-11 18:32 ` Volodymyr Babchuk
2019-09-12 18:55 ` Julien Grall
2019-08-23 18:48 ` [Xen-devel] [PATCH 5/5] xen/arm: optee: remove experimental status Volodymyr Babchuk
2019-08-23 19:05 ` Julien Grall
2019-08-23 19:20 ` Volodymyr Babchuk
2019-09-09 21:31 ` Julien Grall
2019-09-11 18:41 ` Volodymyr Babchuk
2019-09-12 19:00 ` Julien Grall
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