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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-06-04 at 14:46 +0100, Stefano Stabellini wrote:
> 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).

OK. So should we switch to WRITEALLOC explicitly or would you make an
argument for switching to WRITEBACK?

Can we get all that in an update changelog with an S-o-b etc please.

Ian.

> 
> 
> > > 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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