Re: [PATCH V2 17/23] xen/ioreq: Introduce domain_has_ioreq_server()

[Jan Beulich <jbeulich@suse.com>]

On 10.11.2020 21:53, Oleksandr wrote:
> 
> On 20.10.20 13:51, Paul Durrant wrote:
> 
> Hi Paul.
> 
> Sorry for the late response.
> 
>>
>>> -----Original Message-----
>>> From: Oleksandr Tyshchenko <olekstysh@gmail.com>
>>> Sent: 15 October 2020 17:44
>>> To: xen-devel@lists.xenproject.org
>>> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>; Stefano Stabellini <sstabellini@kernel.org>;
>>> Julien Grall <julien@xen.org>; Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>; Andrew Cooper
>>> <andrew.cooper3@citrix.com>; George Dunlap <george.dunlap@citrix.com>; Ian Jackson
>>> <iwj@xenproject.org>; Jan Beulich <jbeulich@suse.com>; Wei Liu <wl@xen.org>; Paul Durrant
>>> <paul@xen.org>; Julien Grall <julien.grall@arm.com>
>>> Subject: [PATCH V2 17/23] xen/ioreq: Introduce domain_has_ioreq_server()
>>>
>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>
>>> This patch introduces a helper the main purpose of which is to check
>>> if a domain is using IOREQ server(s).
>>>
>>> On Arm the current benefit is to avoid calling handle_io_completion()
>>> (which implies iterating over all possible IOREQ servers anyway)
>>> on every return in leave_hypervisor_to_guest() if there is no active
>>> servers for the particular domain.
>>> Also this helper will be used by one of the subsequent patches on Arm.
>>>
>>> This involves adding an extra per-domain variable to store the count
>>> of servers in use.
>>>
>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>> CC: Julien Grall <julien.grall@arm.com>
>>>
>>> ---
>>> Please note, this is a split/cleanup/hardening of Julien's PoC:
>>> "Add support for Guest IO forwarding to a device emulator"
>>>
>>> Changes RFC -> V1:
>>>     - new patch
>>>
>>> Changes V1 -> V2:
>>>     - update patch description
>>>     - guard helper with CONFIG_IOREQ_SERVER
>>>     - remove "hvm" prefix
>>>     - modify helper to just return d->arch.hvm.ioreq_server.nr_servers
>>>     - put suitable ASSERT()s
>>>     - use ASSERT(d->ioreq_server.server[id] ? !s : !!s) in set_ioreq_server()
>>>     - remove d->ioreq_server.nr_servers = 0 from hvm_ioreq_init()
>>> ---
>>>   xen/arch/arm/traps.c    | 15 +++++++++------
>>>   xen/common/ioreq.c      |  7 ++++++-
>>>   xen/include/xen/ioreq.h | 14 ++++++++++++++
>>>   xen/include/xen/sched.h |  1 +
>>>   4 files changed, 30 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
>>> index 507c095..a8f5fdf 100644
>>> --- a/xen/arch/arm/traps.c
>>> +++ b/xen/arch/arm/traps.c
>>> @@ -2261,14 +2261,17 @@ static bool check_for_vcpu_work(void)
>>>       struct vcpu *v = current;
>>>
>>>   #ifdef CONFIG_IOREQ_SERVER
>>> -    bool handled;
>>> +    if ( domain_has_ioreq_server(v->domain) )
>>> +    {
>>> +        bool handled;
>>>
>>> -    local_irq_enable();
>>> -    handled = handle_io_completion(v);
>>> -    local_irq_disable();
>>> +        local_irq_enable();
>>> +        handled = handle_io_completion(v);
>>> +        local_irq_disable();
>>>
>>> -    if ( !handled )
>>> -        return true;
>>> +        if ( !handled )
>>> +            return true;
>>> +    }
>>>   #endif
>>>
>>>       if ( likely(!v->arch.need_flush_to_ram) )
>>> diff --git a/xen/common/ioreq.c b/xen/common/ioreq.c
>>> index bcd4961..a72bc0e 100644
>>> --- a/xen/common/ioreq.c
>>> +++ b/xen/common/ioreq.c
>>> @@ -39,9 +39,14 @@ static void set_ioreq_server(struct domain *d, unsigned int id,
>>>                                struct ioreq_server *s)
>>>   {
>>>       ASSERT(id < MAX_NR_IOREQ_SERVERS);
>>> -    ASSERT(!s || !d->ioreq_server.server[id]);
>>> +    ASSERT(d->ioreq_server.server[id] ? !s : !!s);
>> That looks odd. How about ASSERT(!s ^ !d->ioreq_server.server[id])?
> 
> ok, looks like it will work.
> 
> 
>>    Paul
>>
>>>       d->ioreq_server.server[id] = s;
>>> +
>>> +    if ( s )
>>> +        d->ioreq_server.nr_servers++;
>>> +    else
>>> +        d->ioreq_server.nr_servers--;
>>>   }
>>>
>>>   #define GET_IOREQ_SERVER(d, id) \
>>> diff --git a/xen/include/xen/ioreq.h b/xen/include/xen/ioreq.h
>>> index 7b03ab5..0679fef 100644
>>> --- a/xen/include/xen/ioreq.h
>>> +++ b/xen/include/xen/ioreq.h
>>> @@ -55,6 +55,20 @@ struct ioreq_server {
>>>       uint8_t                bufioreq_handling;
>>>   };
>>>
>>> +#ifdef CONFIG_IOREQ_SERVER
>>> +static inline bool domain_has_ioreq_server(const struct domain *d)
>>> +{
>>> +    ASSERT((current->domain == d) || atomic_read(&d->pause_count));
>>> +
>> This seems like an odd place to put such an assertion.
> 
> I might miss something or interpreted incorrectly but these asserts are 
> the result of how I understood the review comment on previous version [1].
> 
> I will copy a comment here for the convenience:
> "This is safe only when d == current->domain and it's not paused,
> or when they're distinct and d is paused. Otherwise the result is
> stale before the caller can inspect it. This wants documenting by
> at least a comment, but perhaps better by suitable ASSERT()s."

The way his reply was worded, I think Paul was wondering about the
place where you put the assertion, not what you actually assert. 

>>> +    return d->ioreq_server.nr_servers;
>>> +}
>>> +#else
>>> +static inline bool domain_has_ioreq_server(const struct domain *d)
>>> +{
>>> +    return false;
>>> +}
>>> +#endif
>>> +
>> Can this be any more compact? E.g.
>>
>> return IS_ENABLED(CONFIG_IOREQ_SERVER) && d->ioreq_server.nr_servers;
>>
>> ?
> I have got a compilation error this way (if CONFIG_IOREQ_SERVER is 
> disabled):
> 
> ...xen/4.14.0+gitAUTOINC+ee22110219-r0/git/xen/include/xen/ioreq.h:62:48: 
> error: ‘const struct domain’ has no member named ‘ioreq_server’
>       return IS_ENABLED(CONFIG_IOREQ_SERVER) && d->ioreq_server.nr_servers;
>                                                  ^
> as domain's ioreq_server struct is guarded by CONFIG_IOREQ_SERVER as well.

The #ifdef is unavoidable here, I agree, but it should be inside
the function's body. There's no need to duplicate the rest of it.

Jan