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

[Boris Ostrovsky]

On 09/21/2017 12:16 PM, Andrew Cooper wrote:
> On 21/09/17 17:00, Boris Ostrovsky wrote:
> > > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > > > ---
> > > > > ÂÂ arch/x86/include/asm/xen/page.h | 11 ++++++++++-
> > > > > ÂÂ arch/x86/xen/mmu_pv.cÂÂÂÂÂÂÂÂÂÂ |Â 4 ++--
> > > > > ÂÂ 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 {
> > > > > ÂÂÂÂÂÂ phys_addr_t paddr;
> > > > > ÂÂ } xpaddr_t;
> > > > > ÂÂ +#ifdef CONFIG_X86_64
> > > > > +#define XEN_PHYSICAL_MASKÂÂÂ ((1UL << 52) - 1)
> > > >
> > > >
> > > > SME is not supported for PV guests but for consistency (and in case sme
> > > > bit somehow gets set)
> > > > #define XEN_PHYSICAL_MASKÂÂÂ __sme_clr(((1UL << 52) - 1))
> > >
> > > Hmm, really? Shouldn't we rather add something like
> > >
> > > BUG_ON(sme_active());
> > >
> > > somewhere?
> >
> > We can do that too.
>
> Please don't do anything to cause Linux to crash if Xen is using SME 
> itself, but leaving all of the PV guest unencrypted.

sme_active() returns true if the *guest* enables it.

Also, if the guest's memory is unencrypted, doesn't this mean that mfns 
that it sees (or, rather, ptes) will not have the SME bit set?

-boris