Re: [Xen-devel] [PATCH v9 08/15] tools/libxc: x86 PV save code

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-04-15 at 12:41 +0100, Andrew Cooper wrote:
> On 15/04/15 12:13, Ian Campbell wrote:
> > On Fri, 2015-04-10 at 18:16 +0100, Andrew Cooper wrote:
> >> Save the x86 PV specific parts of a domain.  This is the X86_PV_INFO 
> >> record,
> >> the P2M_FRAMES, the X86_PV_SHARED_INFO, the three different VCPU context
> >> records, and the MSR records.
> >>
> >> The normalise_page callback used by the common code when writing the 
> >> PAGE_DATA
> >> records, converts MFNs in page tables to PFNs.
> >>
> >> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Acked-by: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> >
> > With one question:
> >
> >> +#ifdef __x86_64__
> >> +            /* 64bit toolstack, 32bit guest.  Expand any INVALID_MFN. */
> >> +            uint32_t s = ((uint32_t *)src)[x];
> >> +
> >> +            dst[x] = s == ~0U ? INVALID_MFN : s;
> >> +#else
> >> +            /* 32bit toolstack, 64bit guest.  Truncate their pointers */
> >> +            dst[x] = ((uint64_t *)src)[x];
> >> +#endif
> > Would it not be better to propagate an error instead of truncating? Or
> > at least log the first instance of such?
> 
> There are valid truncations to be performed, from 64bit INVALID_MFN to
> 32bit INVALID_MFN.
> 
> For the invalid truncations, that is a little harder.  The post-trucated
> values are the ones which are checked for validity (so is at least safe
> from that point of view), and will fail because of a p2m/m2p mismatch,
> completely aborting the migration.
> 
> Truncation will only actually occur if a domain has pages above the 16TB
> (44 bits) boundary, and a 32bit dom0 is simply not going to function on
> a server that size.
> 
> In other words, I don't think there is a practical situation where
> truncation would be an issue.

OK, good to know, thanks.

I wonder if it is worth putting some of that in a comment or perhaps the
commit log?



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