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

Hi Edgar,

On 16/09/2016 18:17, Edgar E. Iglesias wrote:
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;

+    /*
+     * 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:


+     * Shareable for Normal Inner Non_cacheable, Outer Non-cacheable.


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;
        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
If so, perhaps that same reason also applies to p2m_mmio_direct_nc and
both should be non-executable.

I guess you mean your new p2m_mmio_direct_nc and not the current one. If so, I think it is more a safety. Until now, the p2m type was used to share ACPI tables which should not be executable.

I am not sure what would be the implication to unset the NX bit by default. The question to answer before any relaxation is could a potential misbehave guest harm the platform?


Julien Grall

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