Re: [RFC PATCH v1 1/5] vfio-ccw: Move guest_cp storage into common struct

Farhan Ali
2019-06-19T21:14:02Z

On 06/19/2019 04:53 PM, Eric Farman wrote:
>
>
> On 6/19/19 4:13 PM, Farhan Ali wrote:
>>
>>
>> On 06/18/2019 04:23 PM, Eric Farman wrote:
>>> Rather than allocating/freeing a piece of memory every time
>>> we try to figure out how long a CCW chain is, let's use a piece
>>> of memory allocated for each device.
>>>
>>> The io_mutex added with commit 4f76617378ee9 ("vfio-ccw: protect
>>> the I/O region") is held for the duration of the VFIO_CCW_EVENT_IO_REQ
>>> event that accesses/uses this space, so there should be no race
>>> concerns with another CPU attempting an (unexpected) SSCH for the
>>> same device.
>>>
>>> Suggested-by: Cornelia Huck <cohuck@redhat.com>
>>> Signed-off-by: Eric Farman <farman@linux.ibm.com>
>>> ---
>>> Conny, your suggestion [1] did not go unnoticed.  :)
>>>
>>> [1] https://patchwork.kernel.org/comment/22312659/
>>> ---
>>>   drivers/s390/cio/vfio_ccw_cp.c  | 23 ++++-------------------
>>>   drivers/s390/cio/vfio_ccw_cp.h  |  7 +++++++
>>>   drivers/s390/cio/vfio_ccw_drv.c |  7 +++++++
>>>   3 files changed, 18 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/s390/cio/vfio_ccw_cp.c
>>> b/drivers/s390/cio/vfio_ccw_cp.c
>>> index 90d86e1354c1..f358502376be 100644
>>> --- a/drivers/s390/cio/vfio_ccw_cp.c
>>> +++ b/drivers/s390/cio/vfio_ccw_cp.c
>>> @@ -16,12 +16,6 @@
>>>     #include "vfio_ccw_cp.h"
>>>   -/*
>>> - * Max length for ccw chain.
>>> - * XXX: Limit to 256, need to check more?
>>> - */
>>> -#define CCWCHAIN_LEN_MAX    256
>>> -
>>>   struct pfn_array {
>>>       /* Starting guest physical I/O address. */
>>>       unsigned long        pa_iova;
>>> @@ -386,7 +380,7 @@ static void ccwchain_cda_free(struct ccwchain
>>> *chain, int idx)
>>>    */
>>>   static int ccwchain_calc_length(u64 iova, struct channel_program *cp)
>>>   {
>>> -    struct ccw1 *ccw, *p;
>>> +    struct ccw1 *ccw = cp->guest_cp;
>>>       int cnt;
>>>         /*
>>> @@ -394,15 +388,9 @@ static int ccwchain_calc_length(u64 iova, struct
>>> channel_program *cp)
>>>        * Currently the chain length is limited to CCWCHAIN_LEN_MAX (256).
>>>        * So copying 2K is enough (safe).
>>>        */
>>> -    p = ccw = kcalloc(CCWCHAIN_LEN_MAX, sizeof(*ccw), GFP_KERNEL);
>>> -    if (!ccw)
>>> -        return -ENOMEM;
>>> -
>>>       cnt = copy_ccw_from_iova(cp, ccw, iova, CCWCHAIN_LEN_MAX);
>>
>> Just a minor concern, should we clear out cp->guest_cp memory before we
>> do the copying? Given that the ccwchain_calc_length will also call be
>> called during tic handling, it's possible there might be some garbage
>> data in guest_cp, no?
>
> Yeah, they'll be garbage there, but I'm not sure it's a problem. By the
> time we get here again (ccwchain_loop_tic() -> ccwchain_handle_ccw()),
> we'll have saved the relevant CCWs for the first segment. And the
> second time through we'll be copying a fresh 2K from the target of the
> TIC to cp->guest_cp, overwriting all that stale data with new CCWs (and
> new garbage data).
>

Yes, you are right. Please disregard my concern :)

>>
>>
>>> -    if (cnt) {
>>> -        kfree(ccw);
>>> +    if (cnt)
>>>           return cnt;
>>> -    }
>>>         cnt = 0;
>>>       do {
>>> @@ -413,10 +401,8 @@ static int ccwchain_calc_length(u64 iova, struct
>>> channel_program *cp)
>>>            * orb specified one of the unsupported formats, we defer
>>>            * checking for IDAWs in unsupported formats to here.
>>>            */
>>> -        if ((!cp->orb.cmd.c64 || cp->orb.cmd.i2k) && ccw_is_idal(ccw)) {
>>> -            kfree(p);
>>> +        if ((!cp->orb.cmd.c64 || cp->orb.cmd.i2k) && ccw_is_idal(ccw))
>>>               return -EOPNOTSUPP;
>>> -        }
>>>             /*
>>>            * We want to keep counting if the current CCW has the
>>> @@ -435,7 +421,6 @@ static int ccwchain_calc_length(u64 iova, struct
>>> channel_program *cp)
>>>       if (cnt == CCWCHAIN_LEN_MAX + 1)
>>>           cnt = -EINVAL;
>>>   -    kfree(p);
>>>       return cnt;
>>>   }
>>>   @@ -461,7 +446,7 @@ static int ccwchain_handle_ccw(u32 cda, struct
>>> channel_program *cp)
>>>       struct ccwchain *chain;
>>>       int len, ret;
>>>   -    /* Get chain length. */
>>> +    /* Copy the chain from cda to cp, and count the CCWs in it */
>>>       len = ccwchain_calc_length(cda, cp);
>>>       if (len < 0)
>>>           return len;
>>> diff --git a/drivers/s390/cio/vfio_ccw_cp.h
>>> b/drivers/s390/cio/vfio_ccw_cp.h
>>> index 3c20cd208da5..7cdc38049033 100644
>>> --- a/drivers/s390/cio/vfio_ccw_cp.h
>>> +++ b/drivers/s390/cio/vfio_ccw_cp.h
>>> @@ -16,6 +16,12 @@
>>>     #include "orb.h"
>>>   +/*
>>> + * Max length for ccw chain.
>>> + * XXX: Limit to 256, need to check more?
>>> + */
>>> +#define CCWCHAIN_LEN_MAX    256
>>> +
>>>   /**
>>>    * struct channel_program - manage information for channel program
>>>    * @ccwchain_list: list head of ccwchains
>>> @@ -32,6 +38,7 @@ struct channel_program {
>>>       union orb orb;
>>>       struct device *mdev;
>>>       bool initialized;
>>> +    struct ccw1 *guest_cp;
>>>   };
>>>     extern int cp_init(struct channel_program *cp, struct device *mdev,
>>> diff --git a/drivers/s390/cio/vfio_ccw_drv.c
>>> b/drivers/s390/cio/vfio_ccw_drv.c
>>> index 66a66ac1f3d1..34a9a5e3fd36 100644
>>> --- a/drivers/s390/cio/vfio_ccw_drv.c
>>> +++ b/drivers/s390/cio/vfio_ccw_drv.c
>>> @@ -129,6 +129,11 @@ static int vfio_ccw_sch_probe(struct subchannel
>>> *sch)
>>>       if (!private)
>>>           return -ENOMEM;
>>>   +    private->cp.guest_cp = kcalloc(CCWCHAIN_LEN_MAX, sizeof(struct
>>> ccw1),
>>> +                       GFP_KERNEL);
>>> +    if (!private->cp.guest_cp)
>>> +        goto out_free;
>>> +
>>>       private->io_region = kmem_cache_zalloc(vfio_ccw_io_region,
>>>                              GFP_KERNEL | GFP_DMA);
>>>       if (!private->io_region)
>>> @@ -169,6 +174,7 @@ static int vfio_ccw_sch_probe(struct subchannel *sch)
>>>           kmem_cache_free(vfio_ccw_cmd_region, private->cmd_region);
>>>       if (private->io_region)
>>>           kmem_cache_free(vfio_ccw_io_region, private->io_region);
>>> +    kfree(private->cp.guest_cp);
>>>       kfree(private);
>>>       return ret;
>>>   }
>>> @@ -185,6 +191,7 @@ static int vfio_ccw_sch_remove(struct subchannel
>>> *sch)
>>>         kmem_cache_free(vfio_ccw_cmd_region, private->cmd_region);
>>>       kmem_cache_free(vfio_ccw_io_region, private->io_region);
>>> +    kfree(private->cp.guest_cp);
>>>       kfree(private);
>>>         return 0;
>>>
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