Re: [Xen-devel] [PATCH v2] x86: synchronize PCI config space access decoding

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/06/15 09:09, Jan Beulich wrote:
>>>> On 15.06.15 at 17:32, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 15/06/15 15:30, Jan Beulich wrote:
>>> @@ -2439,9 +2434,19 @@ struct hvm_ioreq_server *hvm_select_iore
>>>  
>>>          type = IOREQ_TYPE_PCI_CONFIG;
>>>          addr = ((uint64_t)sbdf << 32) |
>>> -               CF8_ADDR_HI(cf8) |
>>>                 CF8_ADDR_LO(cf8) |
>>>                 (p->addr & 3);
>>> +        /* AMD extended configuration space access? */
>>> +        if ( CF8_ADDR_HI(cf8) &&
>>> +             d->arch.x86_vendor == X86_VENDOR_AMD &&
>>> +             d->arch.x86 >= 0x10 && d->arch.x86 <= 0x17 )
>>> +        {
>>> +            uint64_t msr_val;
>>> +
>>> +            if ( !rdmsr_safe(MSR_AMD64_NB_CFG, msr_val) &&
>> We now have several common paths which read this MSR looking for CF8_EXT.
>>
>> I think it would make sense to probe this on boot and have a
>> cpu_has_amd_cf8_ext rather than repeatedly sampling an off-cpu MSR,
>> although this would require synchronising it across all northbridges in
>> emulate privileged op.
>>
>> Alternatively, it might just be better to unconditionally enable it
>> during startup (as Linux does) and prevent dom0 from playing, which
>> would avoid the need to synchronise updates to it.
> You just repeat what you said for v1, without taking into
> consideration my reply thereto: Us not using this method
> ourselves, we should honor and play by what Dom0 does.

Sorry - I had completely forgotten that this was a v2, and had already
asked this question.

However, hvm_select_ioreq_server() it not a rare function to call, and I
am still concerned with the overhead.

It turns out that MSR_AMD64_NB_CFG is unconditionally RAZ and has all
writes discarded, so no HVM guest will ever be in a position to
legitimately use AMD extended configuration access.

I would recommend instead terminating the access early, over taking the
rdmsr hit.

>
>>> @@ -1787,9 +1790,9 @@ static bool_t pci_cfg_ok(struct domain *
>>>          if ( ro_map && test_bit(machine_bdf, ro_map) )
>>>              return 0;
>>>      }
>>> -    start = currd->arch.pci_cf8 & 0xFF;
>>> +    start |= CF8_ADDR_LO(currd->arch.pci_cf8);
>> This, combined with the change to the callers, looks suspect.
>>
>> The callers are both accesses at cfc, with port&3 being the offset at
>> the port.  This logical or here is combining the base offset to cfc with
>> the destination address requested via the setting in cf8.
>>
>> Is this intentional, and ifso, why?
> It is: First of all you need to consider what start is being used for -
> solely the call to xsm_pci_config_permission(). And there we want
> the precise range of config space fields being accessed, not some
> rough estimate thereof (i.e. the current code is broken in this
> regard, and the fix is even spelled out in the commit message).

I see that it was spelled out in the commit message, but that doesn't
lend itself to explaining why this new behaviour is correct.

I was not aware of this particular behaviour for 8/16 bit reads of
cfc-cff, but experimentally hardware does behave in this manner.

~Andrew

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