Re: memory locking in vhost-vdpa

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 04, 2022 at 09:52:47AM +0800, Jason Wang wrote:
> On Thu, Aug 4, 2022 at 1:46 AM Andrey Zhadchenko
> <andrey.zhadchenko@virtuozzo.com> wrote:
> >
> > Hi!
> >
> > Recently I sent vhost-blk patchset and Stefano suggested to instead join
> > efforts on developing vdpa-blk.
> > I played with it a bit and looks like vdpa itself pins the whole guest
> > memory. Is there a way to control it or reduce pinned amount to the
> > device pages?
> > Looks like even vdpa-sim requires all memory to be pinned [1]. Pinning
> > this much will surely impact guest density.
> 
> It depends on the parent.
> 
> When allocating the vDPA device, the parent can clams it supports
> virtual address then pinning is avoided:
> 
> /**
>  * __vdpa_alloc_device - allocate and initilaize a vDPA device
>  * This allows driver to some prepartion after device is
>  * initialized but before registered.
> ...
>  * @use_va: indicate whether virtual address must be used by this device
>  */
> 
> The only user so far is VDUSE which is a software parent in the
> userspace with a customized swiotlb for kernel drivers.
> 
> Simulator came before this feature so we stick to the pinning method,
> technically we can switch to use the va mode, but it might have some
> performance impact (mostly the copy_from|to_user()).
> 
> This option might be useful for hardware parent if PRI or device page
> fault is supported in the future.
> 
> Thanks

Well VDUSE has this funky bounce buffer design. It works but
is costly performance wise.



> >
> > Kind regards,
> > Andrey
> >
> > [1] https://bugzilla.redhat.com/show_bug.cgi?id=1868535
> >

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