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Re: [Xen-devel] [PATCH for-4.12] x86/hvm: Fix hvm_cr4_guest_valid_bits() for PVH dom0
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 28 Jan 2019 15:36:22 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- Delivery-date: Mon, 28 Jan 2019 15:37:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 28/01/2019 15:22, Jan Beulich wrote:
>>>> 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).
The PVH Dom0 angle is simply that it is wrong to audit against the host
policy.
The logic from c/s 9d2efbafb8 is a vestigial remnant of PVHv1, because
HVM domains never had the "pass through host CPUID values" logic which
PV guests used to have. Now that all domains have a cpuid policy, this
logic is no longer necessary.
>
>> 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.
Hmm - perhaps not. As I said, I'm not aware of any differences between
the host max policy and the hvm max policy when it comes to the subset
of bits selected by this logic.
~Andrew
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