Re: [Xen-devel] [PATCH XEN RFC] libxc: use an uncached mapping of guest ram in domain builder

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-12-18 at 16:14 -0500, Konrad Rzeszutek Wilk wrote:
> On Wed, Dec 18, 2013 at 05:30:47PM +0000, Ian Campbell wrote:
> > VERY MUCH A WIP.
> > 
> > On ARM guest OSes are started with MMU and Caches disables (as they are on
> > native) however caching is enabled in the domain running the builder and
> > therefore we must use an explcitly uncached mapping, otherwise when the 
> > guest
> > starts running it may not see them.
> > 
> > Cache flushes are not sufficient because there is a race between the flush 
> > and
> > the unmap, where the processor may speculatively fill a cache line. Thanks 
> > to
> > Catalin Marinas and Marc Zyngier for pointing this out.
> > 
> > Therefore use the newly introduced IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED on 
> > ARM
> > when mapping guest memory from xc_dom_* (which means xc_dom_boot_domU_map in
> > practice).  This avoids issues with the processor dirtying cache lines while
> > the guest will then fail to see because it starts with MMU and caches
> > disabled.
> 
> Why not make the default IOCTL_PRIVCMD_MMAPBATCH_V2 when running under ARM
> do this?

For the same reason as I explained in your equivalent reply to the Linux
patch...



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel