Re: [Xen-devel] [PATCH 13/13] x86/domctl: Implement XEN_DOMCTL_set_cpumsr_policy

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/07/18 10:28, Jan Beulich wrote:
>
>>>> +    /*
>>>> +     * Audit was successful.  Replace existing policies, leaving the old
>>>> +     * policies to be freed.
>>>> +     */
>>>> +    SWAP(new.cp, d->arch.cpuid);
>>>> +    SWAP(new.dp, d->arch.msr);
>>>> +    SWAP(new.vp, v->arch.msr);
>>>> +
>>>> +    /* Merge the (now audited) vCPU MSRs into every other 
>>>> msr_vcpu_policy. */
>>>> +    for ( ; v; v = v->next_in_list )
>>> This open-coded almost-for_each_domain() doesn't look very nice.
>> ITYM for_each_vcpu()
> Oops, of course.
>
>> And yes, but for_each_vcpu() is wrong to use here, and we don't have a
>> for_each_vcpu_other_than_0() helper.
> Perhaps still better to do
>
>     for_each_vcpu(d, v)
>     {
>         if ( v->vcpu_id == 0 )
>             continue;
>         ...
>     }
>
> ?

Do you think that looks cleaner? I doubt the compiler can optimise that
into my version.

>
>>>> +    {
>>>> +        /* XXX - Figure out how to avoid a TOCTOU race here.  XLAT area? 
>>>> */
>>>> +        if ( (ret = x86_msr_copy_from_buffer(
>>>> +                  NULL, v->arch.msr, xdpc->msr_policy, xdpc->nr_msrs, 
>>>> NULL)) )
>>> Why can't you go from vCPU 0's v->arch.msr here, which is the copied-in
>>> (and sanitized) representation already? Also, is it really a good idea to
>>> assume all vCPU-s have the same policies?
>> There are multiple colliding issues which lead to this code, but as
>> several people have pointed out, its probably over the top.
>>
>> First, as to the same policy.  This hypercall can currently only be used
>> before the vcpu has started executing.
>>
>> As such, it is setting the init state of the MSRs from the guests point
>> of view, and there is exactly one MSR I'm aware of which has an init
>> value which depends on the core (that being APIC_BASE.BSP which can
>> trivially be handled in Xen).  All other MSRs have identical init state
>> AFAICT, and I don't want to create an interface which makes it easy to
>> accidentally end up with wrong values.
> So what about migration? There are certainly differing incoming values
> there. Of course there's the MSRs restore record, but no atomic sanity
> check between those and the policy here is possible.

Migration is still a problem.  This CPUID/MSR work is the next step on
the path to fixing the "state before policy" problem we've got when
restoring a guest.

Once we have a working CPUID and R/O MSR configuration "blob" which the
toolstack can manipulate, we can (in Xen) require that the toolstack
provide the blob before memory and R/W register state.

When we get to that point, the toolstack shall call
DOMCTL_get_cpumsr_policy (modulo whatever plan I device to fix our R/W
MSR from the VCPU state problem) and place this ahead of the main
memory/register state in the migration stream.

The receiving side will either feed the blob back to Xen, or fabricate
the policy out of thin air at this point.  The current behaviour is to
fabricate a policy out of thin air after all migration stream is
complete, but before unpausing the domain.

~Andrew

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