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From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Miller <davem@davemloft.net>,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	peterx@redhat.com, linux-mm@kvack.org,
	linux-arm-kernel@lists.infradead.org,
	linux-parisc@vger.kernel.org
Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
Date: Thu, 14 Mar 2019 21:49:03 +0800	[thread overview]
Message-ID: <74015196-2e18-4fe0-50ac-0c9d497315c7@redhat.com> (raw)
In-Reply-To: <20190314064004-mutt-send-email-mst@kernel.org>


On 2019/3/14 下午6:42, Michael S. Tsirkin wrote:
>>>>> Which means after we fix vhost to add the flush_dcache_page after
>>>>> kunmap, Parisc will get a double hit (but it also means Parisc
>>>>> was the only one of those archs needed explicit cache flushes,
>>>>> where vhost worked correctly so far.. so it kinds of proofs your
>>>>> point of giving up being the safe choice).
>>>> What double hit?  If there's no cache to flush then cache flush is
>>>> a no-op.  It's also a highly piplineable no-op because the CPU has
>>>> the L1 cache within easy reach.  The only event when flush takes a
>>>> large amount time is if we actually have dirty data to write back
>>>> to main memory.
>>> I've heard people complaining that on some microarchitectures even
>>> no-op cache flushes are relatively expensive.  Don't ask me why,
>>> but if we can easily avoid double flushes we should do that.
>> It's still not entirely free for us.  Our internal cache line is around
>> 32 bytes (some have 16 and some have 64) but that means we need 128
>> flushes for a page ... we definitely can't pipeline them all.  So I
>> agree duplicate flush elimination would be a small improvement.
>>
>> James
> I suspect we'll keep the copyXuser path around for 32 bit anyway -
> right Jason?


Yes since we don't want to slow down 32bit.

Thanks


> So we can also keep using that on parisc...
>
> -- 

  reply	other threads:[~2019-03-14 13:49 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-06  7:18 [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Jason Wang
2019-03-06  7:18 ` [RFC PATCH V2 1/5] vhost: generalize adding used elem Jason Wang
2019-03-06  7:18 ` [RFC PATCH V2 2/5] vhost: fine grain userspace memory accessors Jason Wang
2019-03-06 10:45   ` Christophe de Dinechin
2019-03-07  2:38     ` Jason Wang
2019-03-06  7:18 ` [RFC PATCH V2 3/5] vhost: rename vq_iotlb_prefetch() to vq_meta_prefetch() Jason Wang
2019-03-06  7:18 ` [RFC PATCH V2 4/5] vhost: introduce helpers to get the size of metadata area Jason Wang
2019-03-06 10:56   ` Christophe de Dinechin
2019-03-07  2:40     ` Jason Wang
2019-03-06 18:43   ` Souptick Joarder
2019-03-07  2:42     ` Jason Wang
2019-03-06  7:18 ` [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Jason Wang
2019-03-06 16:31   ` Michael S. Tsirkin
2019-03-07  2:45     ` Jason Wang
2019-03-07 15:34       ` Michael S. Tsirkin
2019-03-07 19:09         ` Jerome Glisse
2019-03-07 19:38           ` Andrea Arcangeli
2019-03-07 20:17             ` Jerome Glisse
2019-03-07 21:27               ` Andrea Arcangeli
2019-03-08  9:13                 ` Jason Wang
2019-03-08 19:11                   ` Andrea Arcangeli
2019-03-11  7:21                     ` Jason Wang
2019-03-11 14:45                 ` Jan Kara
2019-03-08  8:31         ` Jason Wang
2019-03-07 15:47   ` Michael S. Tsirkin
2019-03-07 17:56     ` Michael S. Tsirkin
2019-03-07 19:16       ` Andrea Arcangeli
2019-03-08  8:50         ` Jason Wang
2019-03-08 14:58           ` Jerome Glisse
2019-03-11  7:18             ` Jason Wang
2019-03-08 19:48           ` Andrea Arcangeli
2019-03-08 20:06             ` Jerome Glisse
2019-03-11  7:40             ` Jason Wang
2019-03-11 12:48               ` Michael S. Tsirkin
2019-03-11 13:43                 ` Andrea Arcangeli
2019-03-12  2:56                   ` Jason Wang
2019-03-12  3:51                     ` Michael S. Tsirkin
2019-03-12  2:52                 ` Jason Wang
2019-03-12  3:50                   ` Michael S. Tsirkin
2019-03-12  7:15                     ` Jason Wang
2019-03-07 19:17       ` Jerome Glisse
2019-03-08  2:21         ` Michael S. Tsirkin
2019-03-08  2:55           ` Jerome Glisse
2019-03-08  3:16             ` Michael S. Tsirkin
2019-03-08  3:40               ` Jerome Glisse
2019-03-08  3:43                 ` Michael S. Tsirkin
2019-03-08  3:45                   ` Jerome Glisse
2019-03-08  9:15                     ` Jason Wang
2019-03-08  8:58         ` Jason Wang
2019-03-08 12:56           ` Michael S. Tsirkin
2019-03-08 15:02             ` Jerome Glisse
2019-03-08 19:13           ` Andrea Arcangeli
2019-03-08 14:12 ` [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Christoph Hellwig
2019-03-11  7:13   ` Jason Wang
2019-03-11 13:59     ` Michael S. Tsirkin
2019-03-11 18:14       ` David Miller
2019-03-12  2:59         ` Jason Wang
2019-03-12  3:52           ` Michael S. Tsirkin
2019-03-12  7:17             ` Jason Wang
2019-03-12 11:54               ` Michael S. Tsirkin
2019-03-12 15:46                 ` James Bottomley
2019-03-12 20:04                   ` Andrea Arcangeli
2019-03-12 20:53                     ` James Bottomley
2019-03-12 21:11                       ` Andrea Arcangeli
2019-03-12 21:19                         ` James Bottomley
2019-03-12 21:53                           ` Andrea Arcangeli
2019-03-12 22:02                             ` James Bottomley
2019-03-12 22:50                               ` Andrea Arcangeli
2019-03-12 22:57                                 ` James Bottomley
2019-03-13 16:05                       ` Christoph Hellwig
2019-03-13 16:37                         ` James Bottomley
2019-03-14 10:42                           ` Michael S. Tsirkin
2019-03-14 13:49                             ` Jason Wang [this message]
2019-03-14 19:33                               ` Andrea Arcangeli
2019-03-15  4:39                                 ` Jason Wang
2019-03-12  5:14           ` James Bottomley
2019-03-12  7:51             ` Jason Wang
2019-03-12  7:53               ` Jason Wang

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