Re: [PATCH V3 01/23] x86/ioreq: Prepare IOREQ feature for making it common

[Oleksandr <olekstysh@gmail.com>]

On 01.12.20 13:03, Alex Bennée wrote:

Hi Alex

> Oleksandr Tyshchenko <olekstysh@gmail.com> writes:
>
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> As a lot of x86 code can be re-used on Arm later on, this
>> patch makes some preparation to x86/hvm/ioreq.c before moving
>> to the common code. This way we will get a verbatim copy
> <snip>
>> It worth mentioning that a code which checks the return value of
>> p2m_set_ioreq_server() in hvm_map_mem_type_to_ioreq_server() was
>> folded into arch_ioreq_server_map_mem_type() for the clear split.
>> So the p2m_change_entry_type_global() is called with ioreq_server
>> lock held.
> <snip>
>>   
>> +/* Called with ioreq_server lock held */
>> +int arch_ioreq_server_map_mem_type(struct domain *d,
>> +                                   struct hvm_ioreq_server *s,
>> +                                   uint32_t flags)
>> +{
>> +    int rc = p2m_set_ioreq_server(d, flags, s);
>> +
>> +    if ( rc == 0 && flags == 0 )
>> +    {
>> +        const struct p2m_domain *p2m = p2m_get_hostp2m(d);
>> +
>> +        if ( read_atomic(&p2m->ioreq.entry_count) )
>> +            p2m_change_entry_type_global(d, p2m_ioreq_server, p2m_ram_rw);
>> +    }
>> +
>> +    return rc;
>> +}
>> +
>>   /*
>>    * Map or unmap an ioreq server to specific memory type. For now, only
>>    * HVMMEM_ioreq_server is supported, and in the future new types can be
>> @@ -1112,19 +1155,11 @@ int hvm_map_mem_type_to_ioreq_server(struct domain *d, ioservid_t id,
>>       if ( s->emulator != current->domain )
>>           goto out;
>>   
>> -    rc = p2m_set_ioreq_server(d, flags, s);
>> +    rc = arch_ioreq_server_map_mem_type(d, s, flags);
>>   
>>    out:
>>       spin_unlock_recursive(&d->arch.hvm.ioreq_server.lock);
>>   
>> -    if ( rc == 0 && flags == 0 )
>> -    {
>> -        struct p2m_domain *p2m = p2m_get_hostp2m(d);
>> -
>> -        if ( read_atomic(&p2m->ioreq.entry_count) )
>> -            p2m_change_entry_type_global(d, p2m_ioreq_server, p2m_ram_rw);
>> -    }
>> -
> It should be noted that p2m holds it's own lock but I'm unfamiliar with
> Xen's locking architecture. Is there anything that prevents another vCPU
> accessing a page that is also being used my ioreq on the first vCPU?
I am not sure that I would be able to provide reasonable explanations here.
All what I understand is that p2m_change_entry_type_global() x86 
specific (we don't have p2m_ioreq_server concept on Arm) and should 
remain as such (not exposed to the common code).
IIRC, I raised a question during V2 review whether we could have ioreq 
server lock around the call to p2m_change_entry_type_global() and didn't 
get objections. I may mistake, but looks like the lock being used
in p2m_change_entry_type_global() is yet another lock for protecting 
page table operations, so unlikely we could get into the trouble calling 
this function with the ioreq server lock held.


>
> Assuming that deadlock isn't a possibility to my relatively untrained
> eye this looks good to me:
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

Thank you.


-- 
Regards,

Oleksandr Tyshchenko