From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
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,
Jerome Glisse <jglisse@redhat.com>
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address
Date: Mon, 11 Mar 2019 23:51:25 -0400 [thread overview]
Message-ID: <20190311235100-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <4979eed5-9e3f-5ee0-f4f4-1a5e2a839b21@redhat.com>
On Tue, Mar 12, 2019 at 10:56:20AM +0800, Jason Wang wrote:
>
> On 2019/3/11 下午9:43, Andrea Arcangeli wrote:
> > On Mon, Mar 11, 2019 at 08:48:37AM -0400, Michael S. Tsirkin wrote:
> > > Using copyXuser is better I guess.
> > It certainly would be faster there, but I don't think it's needed if
> > that would be the only use case left that justifies supporting two
> > different models. On small 32bit systems with little RAM kmap won't
> > perform measurably different on 32bit or 64bit systems. If the 32bit
> > host has a lot of ram it all gets slow anyway at accessing RAM above
> > the direct mapping, if compared to 64bit host kernels, it's not just
> > an issue for vhost + mmu notifier + kmap and the best way to optimize
> > things is to run 64bit host kernels.
> >
> > Like Christoph pointed out, the main use case for retaining the
> > copy-user model would be CPUs with virtually indexed not physically
> > tagged data caches (they'll still suffer from the spectre-v1 fix,
> > although I exclude they have to suffer the SMAP
> > slowdown/feature). Those may require some additional flushing than the
> > current copy-user model requires.
> >
> > As a rule of thumb any arch where copy_user_page doesn't define as
> > copy_page will require some additional cache flushing after the
> > kmap. Supposedly with vmap, the vmap layer should have taken care of
> > that (I didn't verify that yet).
>
>
> vmap_page_range()/free_unmap_vmap_area() will call
> fluch_cache_vmap()/flush_cache_vunmap(). So vmap layer should be ok.
>
> Thanks
You only unmap from mmu notifier though.
You don't do it after any access.
>
> >
> > There are some accessories like copy_to_user_page()
> > copy_from_user_page() that could work and obviously defines to raw
> > memcpy on x86 (the main cons is they don't provide word granular
> > access) and at least on sparc they're tailored to ptrace assumptions
> > so then we'd need to evaluate what happens if this is used outside of
> > ptrace context. kmap has been used generally either to access whole
> > pages (i.e. copy_user_page), so ptrace may actually be the only use
> > case with subpage granularity access.
> >
> > #define copy_to_user_page(vma, page, vaddr, dst, src, len) \
> > do { \
> > flush_cache_page(vma, vaddr, page_to_pfn(page)); \
> > memcpy(dst, src, len); \
> > flush_ptrace_access(vma, page, vaddr, src, len, 0); \
> > } while (0)
> >
> > So I wouldn't rule out the need for a dual model, until we solve how
> > to run this stable on non-x86 arches with not physically tagged
> > caches.
> >
> > Thanks,
> > Andrea
next prev parent reply other threads:[~2019-03-12 3:51 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 [this message]
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
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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