Re: [Xen-devel] [PATCH] xen: support 52 bit physical addresses in pv guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 09/21/2017 10:41 AM, Juergen Gross wrote:
> On 21/09/17 16:14, Boris Ostrovsky wrote:
> > On 09/21/2017 04:01 AM, Juergen Gross wrote:
> > > Physical addresses on processors supporting 5 level paging can be up to
> > > 52 bits wide. For a Xen pv guest running on such a machine those
> > > physical addresses have to be supported in order to be able to use any
> > > memory on the machine even if the guest itself does not support 5 level
> > > paging.
> > >
> > > So when reading/writing a MFN from/to a pte don't use the kernel's
> > > PTE_PFN_MASK but a new XEN_PTE_MFN_MASK allowing full 40 bit wide MFNs.
> > full 52 bits?
> The MFN mask is only 40 bits. This plus the 12 bits page offset are 52
> bits of machine address width.
Oh, right --- frames, not addresses.

> > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > ---
> > >    arch/x86/include/asm/xen/page.h | 11 ++++++++++-
> > >    arch/x86/xen/mmu_pv.c           |  4 ++--
> > >    2 files changed, 12 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/xen/page.h
> > > b/arch/x86/include/asm/xen/page.h
> > > index 07b6531813c4..bcb8b193c8d1 100644
> > > --- a/arch/x86/include/asm/xen/page.h
> > > +++ b/arch/x86/include/asm/xen/page.h
> > > @@ -26,6 +26,15 @@ typedef struct xpaddr {
> > >        phys_addr_t paddr;
> > >    } xpaddr_t;
> > >    +#ifdef CONFIG_X86_64
> > > +#define XEN_PHYSICAL_MASK    ((1UL << 52) - 1)
> >
> > SME is not supported for PV guests but for consistency (and in case sme
> > bit somehow gets set)
> > #define XEN_PHYSICAL_MASK    __sme_clr(((1UL << 52) - 1))
> Hmm, really? Shouldn't we rather add something like
>
> BUG_ON(sme_active());
>
> somewhere?
We can do that too.

> > But the real question that I have is whether this patch is sufficient.
> > We are trying to preserve more bits in mfn but then this mfn is used,
> > say, in pte_pfn_to_mfn() to build a pte. Can we be sure that the pte
> > won't be stripped of higher bits in native code (again, as an example,
> > native_make_pte()) because we are compiled with 5LEVEL?
> native_make_pte() just encapsulates pte_t. It doesn't modify the value
> of the pte at all.
It's an interface though and so we don't know what is (or can) happen 
under it.
(I also obviously meant "because we are *not* compiled with 5LEVEL")

> Physical address bits are only ever masked away via PTE_PFN_MASK and I
> haven't found any place where it is used for a MFN other than those I
> touched in this patch.

OK.

-boris

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