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Re: [PATCH] virtio_console: Stop doing DMA on the stack

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  Re: [PATCH] virtio_console: Stop doing DMA on the stack "Michael S. Tsirkin" <mst@redhat.com> - 2016-09-09 20:20 +0200
    Re: [PATCH] virtio_console: Stop doing DMA on the stack Ingo Molnar <mingo@kernel.org> - 2016-09-10 06:40 +0200

#1480242 — Re: [PATCH] virtio_console: Stop doing DMA on the stack

On Thu, Sep 08, 2016 at 08:49:43AM +0200, Ingo Molnar wrote:
> 
> * Amit Shah <amit.shah@redhat.com> wrote:
> 
> > On (Tue) 30 Aug 2016 [08:04:15], Andy Lutomirski wrote:
> > > virtio_console uses a small DMA buffer for control requests.  Move
> > > that buffer into heap memory.
> > > 
> > > Doing virtio DMA on the stack is normally okay on non-DMA-API virtio
> > > systems (which is currently most of them), but it breaks completely
> > > if the stack is virtually mapped.
> > > 
> > > Tested by typing both directions using picocom aimed at /dev/hvc0.
> > > 
> > > Signed-off-by: Andy Lutomirski <luto@kernel.org>
> > 
> > Looks fine,
> > 
> > Reviewed-by: Amit Shah <amit.shah@redhat.com>
> > 
> > > ---
> > > 
> > > Hi all-
> > > 
> > > This is currently broken in tip:x86/asm.  If you (Amit) like this patch,
> > > would it make sense for Ingo to add it to -tip?
> > 
> > Yes, I'm fine with that.
> 
> Thanks! FYI, this patch now lives as:
> 
>   9472fe7040bb ("virtio_console: Stop doing DMA on the stack")
> 
> in tip:x86/asm, and is targeted for a v4.9 merge.
> 
> Thanks,
> 
> 	Ingo

Thinking about it, maybe we should put it in 4.8
after all, for benefit of systems using DMA API with virtio.
Thoughts?

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#1480502

* Michael S. Tsirkin <mst@redhat.com> wrote:

> On Thu, Sep 08, 2016 at 08:49:43AM +0200, Ingo Molnar wrote:
> > 
> > * Amit Shah <amit.shah@redhat.com> wrote:
> > 
> > > On (Tue) 30 Aug 2016 [08:04:15], Andy Lutomirski wrote:
> > > > virtio_console uses a small DMA buffer for control requests.  Move
> > > > that buffer into heap memory.
> > > > 
> > > > Doing virtio DMA on the stack is normally okay on non-DMA-API virtio
> > > > systems (which is currently most of them), but it breaks completely
> > > > if the stack is virtually mapped.
> > > > 
> > > > Tested by typing both directions using picocom aimed at /dev/hvc0.
> > > > 
> > > > Signed-off-by: Andy Lutomirski <luto@kernel.org>
> > > 
> > > Looks fine,
> > > 
> > > Reviewed-by: Amit Shah <amit.shah@redhat.com>
> > > 
> > > > ---
> > > > 
> > > > Hi all-
> > > > 
> > > > This is currently broken in tip:x86/asm.  If you (Amit) like this patch,
> > > > would it make sense for Ingo to add it to -tip?
> > > 
> > > Yes, I'm fine with that.
> > 
> > Thanks! FYI, this patch now lives as:
> > 
> >   9472fe7040bb ("virtio_console: Stop doing DMA on the stack")
> > 
> > in tip:x86/asm, and is targeted for a v4.9 merge.
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> Thinking about it, maybe we should put it in 4.8
> after all, for benefit of systems using DMA API with virtio.
> Thoughts?

So CONFIG_VMAP_STACK=y is only going to be enabled for v4.9 - the enabling commit 
is part of tip:x86/asm as well. So AFAICS the commit is not strictly needed for 
v4.8.

Thanks,

	Ingo

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