Re: [Xen-devel] [PATCH v3 1/6] xen/arm: p2m: Add support for normal non-cacheable memory

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>]

On Fri, Sep 16, 2016 at 04:21:12PM +0200, Julien Grall wrote:
> Hi Edgar,
> 
> On 07/09/2016 08:56, Edgar E. Iglesias wrote:
> >From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>
> >
> >Add support for describing normal non-cacheable memory.
> >
> >Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxxxxx>
> >---
> > xen/arch/arm/p2m.c         | 18 ++++++++++++++++++
> > xen/include/asm-arm/p2m.h  |  1 +
> > xen/include/asm-arm/page.h |  1 +
> > 3 files changed, 20 insertions(+)
> >
> >diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> >index b648a9d..bfef77b 100644
> >--- a/xen/arch/arm/p2m.c
> >+++ b/xen/arch/arm/p2m.c
> >@@ -282,6 +282,7 @@ static void p2m_set_permission(lpae_t *e, p2m_type_t t, 
> >p2m_access_t a)
> >     /* First apply type permissions */
> >     switch ( t )
> >     {
> >+    case p2m_mem_nc:
> >     case p2m_ram_rw:
> >         e->p2m.xn = 0;
> >         e->p2m.write = 1;
> >@@ -376,6 +377,23 @@ static lpae_t mfn_to_p2m_entry(mfn_t mfn, p2m_type_t t, 
> >p2m_access_t a)
> >         e.p2m.sh = LPAE_SH_OUTER;
> >         break;
> >
> >+    /*
> >+     * ARM ARM: Overlaying the shareability attribute (DDI
> >+     * 0406C.b B3-1376 to 1377)
> >+     *
> >+     * A memory region with a resultant memory type attribute of Normal,
> >+     * and a resultant cacheability attribute of Inner Non-cacheable,
> >+     * Outer Non-cacheable, must have a resultant shareability attribute
> >+     * of Outer Shareable, otherwise shareability is UNPREDICTABLE.
> >+     *
> >+     * On ARMv8 sharability is ignored and explicitly treated as Outer
> 
> I know you copied it from mfn_to_xen_entry, but can we fixed the copy with:
> 
> s/sharability/shareability/
> 
> >+     * Shareable for Normal Inner Non_cacheable, Outer Non-cacheable.
> 
> s/_/-/
> 
> Also I would like to see a spec reference for the ARMv8. I think it is the
> note in D4-1788 ARM DDI 0487A.j.
> 
> >+     */
> >+    case p2m_mem_nc:
> >+        e.p2m.mattr = MATTR_MEM_NC;
> >+        e.p2m.sh = LPAE_SH_OUTER;
> >+        break;
> >+
> >     default:
> >         e.p2m.mattr = MATTR_MEM;
> >         e.p2m.sh = LPAE_SH_INNER;
> >diff --git a/xen/include/asm-arm/p2m.h b/xen/include/asm-arm/p2m.h
> >index 53c4d78..b012d50 100644
> >--- a/xen/include/asm-arm/p2m.h
> >+++ b/xen/include/asm-arm/p2m.h
> >@@ -93,6 +93,7 @@ typedef enum {
> >     p2m_ram_ro,         /* Read-only; writes are silently dropped */
> >     p2m_mmio_direct_nc, /* Read/write mapping of genuine MMIO area 
> > non-cacheable */
> >     p2m_mmio_direct_c,  /* Read/write mapping of genuine MMIO area 
> > cacheable */
> >+    p2m_mem_nc,         /* Read/write mapping of Non-cacheable Memory */
> 
> I find the name a bit confusing. Technically p2m_mem_nc is p2m_mmio_direct_c
> version but non-cacheable.
> 
> I have got the feeling that the naming I used on a recent patch is not
> correct. Because p2m_mmio_direct_nc is not doing what is expect (i.e mapping
> non-cacheable). It maps with the device memory attribute.
> 
> Maybe we should rename p2m_mmio_direct_nc to p2m_mmio_direct_dev (because it
> will use the device memory attribute) and then use p2m_mmio_direct_nc for
> your purpose.
> 
> Any opinions?


Something that shows up after doing the rename and otherwise keeping the
patch is that we treat the XN bit differently for p2m_mmio_direct_nc
and p2m_mmio_direct_c.

Is there any reason why we can't allow execution for p2m_mmio_direct_c
mappings?
If so, perhaps that same reason also applies to p2m_mmio_direct_nc and
both should be non-executable.

I think there are use-cases for running guests that will want to
execute from on chip memories. But we can cross that bridge when
we come to it...

Cheers,
Edgar








> 
> 
> >     p2m_map_foreign,    /* Ram pages from foreign domain */
> >     p2m_grant_map_rw,   /* Read/write grant mapping */
> >     p2m_grant_map_ro,   /* Read-only grant mapping */
> >diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
> >index 05d9f82..9adc973 100644
> >--- a/xen/include/asm-arm/page.h
> >+++ b/xen/include/asm-arm/page.h
> >@@ -73,6 +73,7 @@
> >  *
> >  */
> > #define MATTR_DEV     0x1
> >+#define MATTR_MEM_NC  0x5
> > #define MATTR_MEM     0xf
> >
> > /* Flags for get_page_from_gva, gvirt_to_maddr etc */
> >
> 
> Regards,
> 
> -- 
> Julien Grall

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