Re: [Xen-devel] [PATCH] xen/arm: define PAGE_HYPERVISOR as BUFFERABLE

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 4 Jun 2013, Ian Campbell wrote:
> On Tue, 2013-06-04 at 14:29 +0100, Stefano Stabellini wrote:
> > On Fri, 10 May 2013, Stefano Stabellini wrote:
> > > Use stage 1 attribute indexes for PAGE_HYPERVISOR, the appriopriate one
> > > for normal memory hypervisor mappings would be BUFFERABLE.
> > > 
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > >
> > > diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
> > > index fd6946e..13f76fc 100644
> > > --- a/xen/include/asm-arm/page.h
> > > +++ b/xen/include/asm-arm/page.h
> > > @@ -58,7 +58,7 @@
> > >  #define DEV_WC        BUFFERABLE
> > >  #define DEV_CACHED    WRITEBACK
> > >  
> > > -#define PAGE_HYPERVISOR         (MATTR_MEM)
> > > +#define PAGE_HYPERVISOR         (BUFFERABLE)
> > >  #define MAP_SMALL_PAGES         PAGE_HYPERVISOR
> > >  
> > >  /*
> >  
> > This patch is completely wrong, fortunately it hasn't been applied.
> 
> Yes, it appears to have slipped through my net, sorry. (But good in this
> case!)
> 
> > 
> > Despite the name, BUFFERABLE means non-cacheable. Mapping a guest page
> > non-cacheable in the hypervisor would cause mismatch in memory
> > attributes.
> > 
> > We should map the guest page as "normal memory", that is WRITEBACK.
> 
> The hypervisor's own memory mappings are using WRITEALLOC not WRITEBACK
> (see mfn_to_xen_entry). Is one or the other wrong?

They are both appropriate. BTW WRITEALLOC is effectively what we have
now (MATTR_MEM is truncated to 0x7 that is WRITEALLOC).


> > diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
> > index fd6946e..13f76fc 100644
> > --- a/xen/include/asm-arm/page.h
> > +++ b/xen/include/asm-arm/page.h
> > @@ -58,7 +58,7 @@
> >  #define DEV_WC        BUFFERABLE
> >  #define DEV_CACHED    WRITEBACK
> >  
> > -#define PAGE_HYPERVISOR         (MATTR_MEM)
> > +#define PAGE_HYPERVISOR         (WRITEBACK)
> >  #define MAP_SMALL_PAGES         PAGE_HYPERVISOR


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