From: Maxime Coquelin <maxime.coquelin@redhat.com>
To: Ilya Maximets <i.maximets@samsung.com>,
dev@dpdk.org, jfreimann@redhat.com, tiwei.bie@intel.com,
zhihong.wang@intel.com, jasowang@redhat.com
Subject: Re: [dpdk-dev] [5/5] vhost: remove useless casts to volatile
Date: Thu, 6 Dec 2018 17:59:00 +0100 [thread overview]
Message-ID: <f2434e1b-2068-3cfc-54ba-dfb2e91a89e9@redhat.com> (raw)
In-Reply-To: <1a4073a9-fc9b-a3c6-5761-7269fe955745@samsung.com>
Hi Ilya,
On 12/5/18 2:52 PM, Ilya Maximets wrote:
> On 05.12.2018 12:49, Maxime Coquelin wrote:
>> Cast to volatile is done when reading avail index and writing
>> the used index. This would not be necessary if proper barriers
>> are used.
>
> 'volatile' and barriers are not really connected. 'volatile' is
> the disabling of the compiler optimizations, while barriers are
> for runtime CPU level optimizations. In general, casts here made
> to force compiler to actually read the value and not cache it
> somehow. In fact that vhost library never writes to avail index,
> "very smart" compiler could drop it at all. None of modern compilers
> will do that for a single operation within a function, so,
> volatiles are not really necessary in current code, but they could
> save some nerves in case of code/compiler changes.
Ok, thanks for the explanation.
Why don't we do the same in Virtio PMD?
> OTOH, IMHO, the main purpose of the casts in current code is
> the self-documenting. Casts forces to pay special attention to
> these variables and reminds that they could be updated in other
> process. Casts allows to understand which variables are local and
> which are shared. I don't think that we should remove them anyway.
It is not only self-documenting, it has an impact on generated code:
>>
>> Now that the read barrier has been added, we can remove these
>> cast to volatile.
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>> ---
>> lib/librte_vhost/virtio_net.c | 7 +++----
>> 1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
>> index 679ce388b..eab1a5b4c 100644
>> --- a/lib/librte_vhost/virtio_net.c
>> +++ b/lib/librte_vhost/virtio_net.c
>> @@ -114,7 +114,7 @@ flush_shadow_used_ring_split(struct virtio_net *dev, struct vhost_virtqueue *vq)
>>
>> vhost_log_cache_sync(dev, vq);
>>
>> - *(volatile uint16_t *)&vq->used->idx += vq->shadow_used_idx;
>> + vq->used->idx += vq->shadow_used_idx;
With cast to volatile:
*(volatile uint16_t *)&vq->used->idx += vq->shadow_used_idx;
35f8: 49 8b 53 10 mov 0x10(%r11),%rdx
vq->shadow_used_idx = 0;
35fc: 31 db xor %ebx,%ebx
*(volatile uint16_t *)&vq->used->idx += vq->shadow_used_idx;
35fe: 0f b7 42 02 movzwl 0x2(%rdx),%eax
3602: 66 41 03 43 70 add 0x70(%r11),%ax
3607: 66 89 42 02 mov %ax,0x2(%rdx)
vq->shadow_used_idx = 0;
Without it:
vq->used->idx += vq->shadow_used_idx;
35f8: 49 8b 43 10 mov 0x10(%r11),%rax
35fc: 41 0f b7 53 70 movzwl 0x70(%r11),%edx
vq->shadow_used_idx = 0;
3601: 31 db xor %ebx,%ebx
vq->used->idx += vq->shadow_used_idx;
3603: 66 01 50 02 add %dx,0x2(%rax)
vq->shadow_used_idx = 0;
If my understanding is correct there is no functional change, but we
save one instruction by removing the cast to volatile.
Thanks,
Maxime
next prev parent reply other threads:[~2018-12-06 16:59 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-05 9:49 [dpdk-dev] [PATCH 0/5] vhost: add missing barriers, remove useless volatiles Maxime Coquelin
2018-12-05 9:49 ` [dpdk-dev] [PATCH 1/5] vhost: enforce avail index and desc read ordering Maxime Coquelin
[not found] ` <CGME20181205113041eucas1p1943b9c13af2fb5b736ba4906b59a9cd5@eucas1p1.samsung.com>
2018-12-05 11:30 ` [dpdk-dev] [1/5] " Ilya Maximets
2018-12-06 4:17 ` Jason Wang
2018-12-06 12:48 ` Ilya Maximets
2018-12-06 13:25 ` Jason Wang
2018-12-06 13:48 ` Michael S. Tsirkin
2018-12-07 14:58 ` Ilya Maximets
2018-12-07 15:44 ` Michael S. Tsirkin
[not found] ` <CGME20181211103848eucas1p10c270ca8997fea8a2f55c2d94d02baea@eucas1p1.samsung.com>
2018-12-11 10:38 ` Ilya Maximets
2018-12-11 14:46 ` Maxime Coquelin
2018-12-05 9:49 ` [dpdk-dev] [PATCH 2/5] vhost: enforce desc flags and content " Maxime Coquelin
[not found] ` <CGME20181205133332eucas1p195b3864ed146403e314d7004d27be285@eucas1p1.samsung.com>
2018-12-05 13:33 ` [dpdk-dev] [2/5] " Ilya Maximets
2018-12-06 4:24 ` Jason Wang
2018-12-06 11:34 ` Ilya Maximets
2018-12-05 9:49 ` [dpdk-dev] [PATCH 3/5] vhost: prefetch descriptor after the read barrier Maxime Coquelin
2018-12-05 9:49 ` [dpdk-dev] [PATCH 4/5] vhost: remove useless prefetch for packed ring descriptor Maxime Coquelin
2018-12-05 9:49 ` [dpdk-dev] [PATCH 5/5] vhost: remove useless casts to volatile Maxime Coquelin
[not found] ` <CGME20181205135231eucas1p1c89281f6525a0fedab4a2fc0d2e21393@eucas1p1.samsung.com>
2018-12-05 13:52 ` [dpdk-dev] [5/5] " Ilya Maximets
2018-12-06 16:59 ` Maxime Coquelin [this message]
2018-12-07 11:16 ` Ilya Maximets
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