Re: [PATCH v2 4/4] KVM: add scatterlist support to xinterface

[Gregory Haskins]

Avi Kivity wrote:
> On 10/02/2009 10:19 PM, Gregory Haskins wrote:
>> This allows a scatter-gather approach to IO, which will be useful for
>> building high performance interfaces, like zero-copy and low-latency
>> copy (avoiding multiple calls to copy_to/from).
>>
>> The interface is based on the existing scatterlist infrastructure.  The
>> caller is expected to pass in a scatterlist with its "dma" field
>> populated with valid GPAs.  The xinterface will then populate each
>> entry by translating the GPA to a page*.
>>
>> The caller signifies completion by simply performing a put_page() on
>> each page returned in the list.
>>
>> Signed-off-by: Gregory Haskins<ghaskins@xxxxxxxxxx>
>> ---
>>
>>   include/linux/kvm_xinterface.h |    4 ++
>>   virt/kvm/xinterface.c          |   72
>> ++++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 76 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/kvm_xinterface.h
>> b/include/linux/kvm_xinterface.h
>> index 684b6f8..eefb575 100644
>> --- a/include/linux/kvm_xinterface.h
>> +++ b/include/linux/kvm_xinterface.h
>> @@ -9,6 +9,7 @@
>>   #include<linux/kref.h>
>>   #include<linux/module.h>
>>   #include<linux/file.h>
>> +#include<linux/scatterlist.h>
>>
>>   struct kvm_xinterface;
>>   struct kvm_xvmap;
>> @@ -36,6 +37,9 @@ struct kvm_xinterface_ops {
>>                       u64 addr,
>>                       unsigned long len,
>>                       unsigned long flags);
>> +    unsigned long (*sgmap)(struct kvm_xinterface *intf,
>> +                   struct scatterlist *sgl, int nents,
>> +                   unsigned long flags);
>>       void (*release)(struct kvm_xinterface *);
>>   };
>>
>> diff --git a/virt/kvm/xinterface.c b/virt/kvm/xinterface.c
>> index c356835..16729f6 100644
>> --- a/virt/kvm/xinterface.c
>> +++ b/virt/kvm/xinterface.c
>> @@ -467,6 +467,77 @@ fail:
>>
>>   }
>>
>> +static unsigned long
>> +xinterface_sgmap(struct kvm_xinterface *intf,
>> +         struct scatterlist *sgl, int nents,
>> +         unsigned long flags)
>> +{
>> +    struct _xinterface     *_intf   = to_intf(intf);
>> +    struct task_struct     *p       = _intf->task;
>> +    struct mm_struct       *mm      = _intf->mm;
>> +    struct kvm             *kvm     = _intf->kvm;
>> +    struct kvm_memory_slot *memslot = NULL;
>> +    bool                    kthread = !current->mm;
>> +    int                     ret;
>> +    struct scatterlist     *sg;
>> +    int                     i;
>> +
>> +    down_read(&kvm->slots_lock);
>> +
>> +    if (kthread)
>> +        use_mm(_intf->mm);
>> +
>> +    for_each_sg(sgl, sg, nents, i) {
>> +        unsigned long           gpa    = sg_dma_address(sg);
>> +        unsigned long           len    = sg_dma_len(sg);
>> +        unsigned long           gfn    = gpa>>  PAGE_SHIFT;
>> +        off_t                   offset = offset_in_page(gpa);
>> +        unsigned long           hva;
>> +        struct page            *pg;
>> +
>> +        /* ensure that we do not have more than one page per entry */
>> +        if ((PAGE_ALIGN(len + offset)>>  PAGE_SHIFT) != 1) {
>> +            ret = -EINVAL;
>> +            break;
>> +        }
>> +
>> +        /* check for a memslot-cache miss */
>> +        if (!memslot
>> +            || gfn<  memslot->base_gfn
>> +            || gfn>= memslot->base_gfn + memslot->npages) {
>> +            memslot = gfn_to_memslot(kvm, gfn);
>> +            if (!memslot) {
>> +                ret = -EFAULT;
>> +                break;
>> +            }
>> +        }
>> +
>> +        hva = (memslot->userspace_addr +
>> +               (gfn - memslot->base_gfn) * PAGE_SIZE);
>> +
>> +        if (kthread || current->mm == mm)
>> +            ret = get_user_pages_fast(hva, 1, 1,&pg);
>> +        else
>> +            ret = get_user_pages(p, mm, hva, 1, 1, 0,&pg, NULL);
>>    
> 
> One of these needs the mm semaphore.

Indeed.  Good catch.

> 
>> +
>> +        if (ret != 1) {
>> +            if (ret>= 0)
>> +                ret = -EFAULT;
>> +            break;
>> +        }
>> +
>> +        sg_set_page(sg, pg, len, offset);
>> +        ret = 0;
>> +    }
>> +
>> +    if (kthread)
>> +        unuse_mm(_intf->mm);
>> +
>> +    up_read(&kvm->slots_lock);
>> +
>> +    return ret;
>> +}
>> +
>>   static void
>>   xinterface_release(struct kvm_xinterface *intf)
>>   {
>> @@ -483,6 +554,7 @@ struct kvm_xinterface_ops _xinterface_ops = {
>>       .copy_from   = xinterface_copy_from,
>>       .vmap        = xinterface_vmap,
>>       .ioevent     = xinterface_ioevent,
>> +    .sgmap       = xinterface_sgmap,
>>       .release     = xinterface_release,
>>   };
>>
>>
>> -- 
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>>    
> 
>

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