Re: [Xen-devel] [PATCH 0/5] x86: properly propagate errors to hypercall callee

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 09, 2011 at 02:20:14PM +0000, Jan Beulich wrote:
> >>> On 09.03.11 at 14:44, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
> > I wonder what the scope of the problem really is. Mostly this cacheattr
> > stuff applies to memory allocated by a graphics driver I suppose, and
> > probably at boot time in dom0. I wonder how the bug was observed during dom0
> > boot given that Xen chooses a default dom0 memory allocation that leaves
> > enough memory free for a decent-sized dom0 SWIOTLB plus some extra slack on
> > top of that. Any idea how the Xen memory pool happened to be entirely empty
> > at the time radeon drm driver caused the superpage shattering to occur?
> 
> This isn't a boot time problem, it's a run time one (and was reported
> to us as such). The driver does allocations (and cache attribute
> changes) based on user mode (X) demands.

What version of radeon Xorg driver is this with? And what radeon card
was this observed with?

> 
> > I'm not against turning the host crash into a guest crash (which I think is
> > typically what is going to happen, although I suppose at least some Linux
> > driver-related mapping/remapping functions can handle failure) as this might
> > be an improvement when starting up non-dom0 driver domains for example. But
> 
> I'm afraid that's not only a question of driver domains doing such.
> With the addition of !is_hvm_domain() to l1_disallow_mask(), any
> page in a HVM guest that its kernel chooses to make non-WB can
> trigger the BUG() currently.
> 
> And, noting just now, there's then a potential collision between
> the kernel and tools/stubdom (qemu-dm) mapping the page - the
> latter, mapping a page WB, would undo what the guest itself may
> have requested earlier - imo the cache attr adjustment shouldn't
> be done if it's not the owner of the page that's doing the mapping
> (and quite probably the cache attr should be inherited by the non-
> owner, though that raises the problem of updating mappings that
> the non-owner may have established before the owner assigned
> the non-default attr).
> 
> > I think we should consider punting a resource error up to the guest as a
> > very bad thing and still WARN_ON or otherwise log the situation to Xen's own
> > console.
> 
> Hmm, possibly.
> 
> Jan
> 
> 
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