Re: [Xen-devel] [PATCH for-4.12] x86/hvm: Fix hvm_cr4_guest_valid_bits() for PVH dom0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.01.19 at 14:56, <andrew.cooper3@xxxxxxxxxx> wrote:
> Before the cpuid_policy logic came along, %cr4 auditing on migrate-in was
> complicated, because at that point no CPUID information had been set for the
> guest.  Auditing against the host CPUID was better than nothing, but not
> ideal.
> 
> Order of information in the migration stream is still an issue (hence we still
> need to keep the restore parameter to cope with a nested virt corner case),
> but since Xen 4.9, the domain starts with the applicable max policy, which is
> a more appropriate upper bound than the host cpuid policy.
> 
> This also makes the fix from c/s 9d2efbafb8 obsolete, as not even dom0 starts
> without a policy.

While I agree with the change itself, I'm struggling to make a connection
from this description to what was actually wrong for PVH Dom0. You
mostly talk about migration, which is not relevant do Dom0 as an object
(and I don't see a connection to domains being migrated by PVH Dom0).

> This ideally wants backporting to Xen 4.9 and later.  I don't think there is
> anything we can reasonably do for 4.8 and earlier.

As (per its title) it affects PVH Dom0 only, is this really something
worth backporting? (But perhaps this clarifies itself once I better
understand the relation between title and description.

Jan



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