Re: [PATCH V1 13/16] xen/ioreq: Make x86's invalidate qemu mapcache handling common

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.09.2020 18:45, Oleksandr wrote:
> 
> On 24.09.20 14:16, Jan Beulich wrote:
> 
> Hi Jan
> 
>> On 22.09.2020 21:32, Oleksandr wrote:
>>> On 16.09.20 11:50, Jan Beulich wrote:
>>>> On 10.09.2020 22:22, Oleksandr Tyshchenko wrote:
>>>>> --- a/xen/common/memory.c
>>>>> +++ b/xen/common/memory.c
>>>>> @@ -1651,6 +1651,11 @@ long do_memory_op(unsigned long cmd, 
>>>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>>            break;
>>>>>        }
>>>>>    
>>>>> +#ifdef CONFIG_IOREQ_SERVER
>>>>> +    if ( op == XENMEM_decrease_reservation )
>>>>> +        curr_d->qemu_mapcache_invalidate = true;
>>>>> +#endif
>>>> I don't see why you put this right into decrease_reservation(). This
>>>> isn't just to avoid the extra conditional, but first and foremost to
>>>> avoid bypassing the earlier return from the function (in the case of
>>>> preemption). In the context of this I wonder whether the ordering of
>>>> operations in hvm_hypercall() is actually correct.
>>> Good point, indeed we may return earlier in case of preemption, I missed
>>> that.
>>> Will move it to decrease_reservation(). But, we may return even earlier
>>> in case of error...
>>> Now I am wondering should we move it to the very beginning of command
>>> processing or not?
>> In _this_ series I'd strongly recommend you keep things working as
>> they are. If independently you think you've found a reason to
>> re-order certain operations, then feel free to send a patch with
>> suitable justification.
> 
> Of course, I will try to retain current behavior.
> 
> 
>>>> I'm also unconvinced curr_d is the right domain in all cases here;
>>>> while this may be a pre-existing issue in principle, I'm afraid it
>>>> gets more pronounced by the logic getting moved to common code.
>>> Sorry I didn't get your concern here.
>> Well, you need to be concerned whose qemu_mapcache_invalidate flag
>> you set.
> May I ask, in what cases the *curr_d* is the right domain?

When a domain does a decrease-reservation on itself. I thought
that's obvious. But perhaps your question was rather meant a to
whether a->domain ever is _not_ the right one?

> We need to make sure that domain is using IOREQ server(s) at least. 
> Hopefully, we have a helper for this
> which is hvm_domain_has_ioreq_server(). Please clarify, anything else I 
> should taking care of?

Nothing I can recall / think of right now, except that the change
may want to come under a different title and with a different
description. As indicated, I don't think this is correct for PVH
Dom0 issuing the request against a HVM DomU, and addressing this
will likely want this moved out of hvm_memory_op() anyway. Of
course an option is to split this into two patches - the proposed
bug fix (perhaps wanting backporting) and then the moving of the
field out of arch.hvm. If you feel uneasy about the bug fix part,
let me know and I (or maybe Roger) will see to put together a
patch.

Jan