Re: [Xen-devel] [PATCH 5/5] x86/HVM: don't setup an intercept handler for IO port 0xcf8 unconditionally

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/01/16 09:12, Jan Beulich wrote:
>>>> On 14.01.16 at 09:35, <roger.pau@xxxxxxxxxx> wrote:
>> El 13/01/16 a les 17.48, Paul Durrant ha escrit:
>>>> -----Original Message-----
>>>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>>> Sent: 13 January 2016 16:39
>>>> To: Paul Durrant; Roger Pau Monne
>>>> Cc: Andrew Cooper; xen-devel@xxxxxxxxxxxxxxxxxxxx 
>>>> Subject: Re: [PATCH 5/5] x86/HVM: don't setup an intercept handler for IO
>>>> port 0xcf8 unconditionally
>>>>
>>>>>>> On 13.01.16 at 13:32, <roger.pau@xxxxxxxxxx> wrote:
>>>>> Only intercept accesses to IO port 0xcf8 if there's at least one IOREQ
>>>>> server, otherwise it makes no sense since the only code that uses the 
>>>>> value
>>>>> stored by hvm_access_cf8 is the IOREQ server.
>>>> Afaict an ioreq server could also attach subsequently - Paul?
>>>>
>>> Indeed, ioreq servers can come and go at any time.
>> Right, then I will have to prevent hvm_access_cf8 from being added if
>> the domain is the hardware domain, otherwise it overlaps with the Dom0
>> passthrough intercept (handle_pvh_io).
> Yes, that indeed makes sense.

Even for the hardware domain, cf8 needs trapping and emulating (although
on a different path).  Being an indirect port pair shared by Xen and
dom0, dom0 cannot use it directly of its own accord.

~Andrew

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