Re: [Qemu-devel] [PATCH v6 2/2] qdisk - hw/block/xen_disk: grant copy implementation

[Paulina Szubarczyk <paulinaszubarczyk@gmail.com>]

On 09/07/2016 10:56 PM, Stefano Stabellini wrote:
> On Wed, 7 Sep 2016, Paulina Szubarczyk wrote:
>> Copy data operated on during request from/to local buffers to/from
>> the grant references.
>>
>> Before grant copy operation local buffers must be allocated what is
>> done by calling ioreq_init_copy_buffers. For the 'read' operation,
>> first, the qemu device invokes the read operation on local buffers
>> and on the completion grant copy is called and buffers are freed.
>> For the 'write' operation grant copy is performed before invoking
>> write by qemu device.
>>
>> A new value 'feature_grant_copy' is added to recognize when the
>> grant copy operation is supported by a guest.
>>
>> Signed-off-by: Paulina Szubarczyk <paulinaszubarczyk@gmail.com>
>> ---
>> Changes since v5:
>> -added checking of every interface in the configure file. Based on
>>   the Roger's comment that xengnttab_map_grant_ref was added prior
>>   xengnttab_grant_copy, thus do not need to be check again here
>>   I dropped this check.
>>
>
> Thank you Paulina, the patch is good. Thanks for your work! Sorry for
> coming in late in the review; I have a couple of minor suggestions
> below.
>
It had not been possible without all the help I have got. I am very 
grateful for it.

> In addition to Anthony's ack, it would be also nice to have Roger's ack
> on this patch.
>
>
>>   configure                   |  55 ++++++++++++++++
>>   hw/block/xen_disk.c         | 157 ++++++++++++++++++++++++++++++++++++++++++--
>>   include/hw/xen/xen_common.h |  14 ++++
>>   3 files changed, 221 insertions(+), 5 deletions(-)
>>
>> diff --git a/configure b/configure
>> index 4b808f9..3f44d38 100755
>> --- a/configure
>> +++ b/configure
>> @@ -1956,6 +1956,61 @@ EOF
>>   /*
>>    * If we have stable libs the we don't want the libxc compat
>>    * layers, regardless of what CFLAGS we may have been given.
>> + *
>> + * Also, check if xengnttab_grant_copy_segment_t is defined and
>> + * grant copy operation is implemented.
>> + */
>> +#undef XC_WANT_COMPAT_EVTCHN_API
>> +#undef XC_WANT_COMPAT_GNTTAB_API
>> +#undef XC_WANT_COMPAT_MAP_FOREIGN_API
>> +#include <xenctrl.h>
>> +#include <xenstore.h>
>> +#include <xenevtchn.h>
>> +#include <xengnttab.h>
>> +#include <xenforeignmemory.h>
>> +#include <stdint.h>
>> +#include <xen/hvm/hvm_info_table.h>
>> +#if !defined(HVM_MAX_VCPUS)
>> +# error HVM_MAX_VCPUS not defined
>> +#endif
>> +int main(void) {
>> +  xc_interface *xc = NULL;
>> +  xenforeignmemory_handle *xfmem;
>> +  xenevtchn_handle *xe;
>> +  xengnttab_handle *xg;
>> +  xen_domain_handle_t handle;
>> +  xengnttab_grant_copy_segment_t* seg = NULL;
>> +
>> +  xs_daemon_open();
>> +
>> +  xc = xc_interface_open(0, 0, 0);
>> +  xc_hvm_set_mem_type(0, 0, HVMMEM_ram_ro, 0, 0);
>> +  xc_domain_add_to_physmap(0, 0, XENMAPSPACE_gmfn, 0, 0);
>> +  xc_hvm_inject_msi(xc, 0, 0xf0000000, 0x00000000);
>> +  xc_hvm_create_ioreq_server(xc, 0, HVM_IOREQSRV_BUFIOREQ_ATOMIC, NULL);
>> +  xc_domain_create(xc, 0, handle, 0, NULL, NULL);
>> +
>> +  xfmem = xenforeignmemory_open(0, 0);
>> +  xenforeignmemory_map(xfmem, 0, 0, 0, 0, 0);
>> +
>> +  xe = xenevtchn_open(0, 0);
>> +  xenevtchn_fd(xe);
>> +
>> +  xg = xengnttab_open(0, 0);
>> +  xengnttab_grant_copy(xg, 0, seg);
>> +
>> +  return 0;
>> +}
>> +EOF
>> +      compile_prog "" "$xen_libs $xen_stable_libs"
>> +    then
>> +    xen_ctrl_version=480
>> +    xen=yes
>> +  elif
>> +      cat > $TMPC <<EOF &&
>> +/*
>> + * If we have stable libs the we don't want the libxc compat
>> + * layers, regardless of what CFLAGS we may have been given.
>>    */
>>   #undef XC_WANT_COMPAT_EVTCHN_API
>>   #undef XC_WANT_COMPAT_GNTTAB_API
>> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
>> index 3b8ad33..3739e13 100644
>> --- a/hw/block/xen_disk.c
>> +++ b/hw/block/xen_disk.c
>> @@ -119,6 +119,9 @@ struct XenBlkDev {
>>       unsigned int        persistent_gnt_count;
>>       unsigned int        max_grants;
>>
>> +    /* Grant copy */
>> +    gboolean            feature_grant_copy;
>> +
>>       /* qemu block driver */
>>       DriveInfo           *dinfo;
>>       BlockBackend        *blk;
>> @@ -489,6 +492,108 @@ static int ioreq_map(struct ioreq *ioreq)
>>       return 0;
>>   }
>>
>> +#if CONFIG_XEN_CTRL_INTERFACE_VERSION >= 480
>
> In general I prefer to avoid this kind of #ifdef in xen_disk.c, but I
> see that Anthony suggested it for a good reason. The only alternartive I
> can think of would be to introduce two static inline functions in
> xen_common.h to set a xengnttab_grant_copy_segment_t seg. But this is
> also OK.
>
The functions take as parameters pointers to struct ioreq defined in 
xen_disk.c which members are used to fill the 
xengnttab_grant_copy_segment_t. There would be some overhead to move the 
functions to the header.
>
>> +static void free_buffers(struct ioreq *ioreq)
>
> Please name this function ioreq_free_copy_buffers to make it clearer
> that has to do with the same buffers initialized below.
>
>
>> +{
>> +    int i;
>> +
>> +    for (i = 0; i < ioreq->v.niov; i++) {
>> +        ioreq->page[i] = NULL;
>> +    }
>> +
>> +    qemu_vfree(ioreq->pages);
>> +}
>> +
>> +static int ioreq_init_copy_buffers(struct ioreq *ioreq)
>> +{
>> +    int i;
>> +
>> +    if (ioreq->v.niov == 0) {
>> +        return 0;
>> +    }
>> +
>> +    ioreq->pages = qemu_memalign(XC_PAGE_SIZE, ioreq->v.niov * XC_PAGE_SIZE);
>> +
>> +    for (i = 0; i < ioreq->v.niov; i++) {
>> +        ioreq->page[i] = ioreq->pages + i * XC_PAGE_SIZE;
>> +        ioreq->v.iov[i].iov_base = ioreq->page[i];
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>> +static int ioreq_copy(struct ioreq *ioreq)
>
> Please name this function in a way that makes it clear that it has
> something to do with grant copies. Like for example ioreq_grant_copy.
>
I will change the names of both functions and remove the blank line 
pointed below.
>
>> +{
>> +    xengnttab_handle *gnt = ioreq->blkdev->xendev.gnttabdev;
>> +    xengnttab_grant_copy_segment_t segs[BLKIF_MAX_SEGMENTS_PER_REQUEST];
>> +    int i, count, rc;
>> +    int64_t file_blk = ioreq->blkdev->file_blk;
>> +
>> +    if (ioreq->v.niov == 0) {
>> +        return 0;
>> +    }
>> +
>> +    count = ioreq->v.niov;
>> +
>> +    for (i = 0; i < count; i++) {
>> +
>> +        if (ioreq->req.operation == BLKIF_OP_READ) {
>> +            segs[i].flags = GNTCOPY_dest_gref;
>> +            segs[i].dest.foreign.ref = ioreq->refs[i];
>> +            segs[i].dest.foreign.domid = ioreq->domids[i];
>> +            segs[i].dest.foreign.offset = ioreq->req.seg[i].first_sect * file_blk;
>> +            segs[i].source.virt = ioreq->v.iov[i].iov_base;
>> +        } else {
>> +            segs[i].flags = GNTCOPY_source_gref;
>> +            segs[i].source.foreign.ref = ioreq->refs[i];
>> +            segs[i].source.foreign.domid = ioreq->domids[i];
>> +            segs[i].source.foreign.offset = ioreq->req.seg[i].first_sect * file_blk;
>> +            segs[i].dest.virt = ioreq->v.iov[i].iov_base;
>> +        }
>> +        segs[i].len = (ioreq->req.seg[i].last_sect
>> +                       - ioreq->req.seg[i].first_sect + 1) * file_blk;
>> +
>> +    }
>> +
>> +    rc = xengnttab_grant_copy(gnt, count, segs);
>> +
>> +    if (rc) {
>> +        xen_be_printf(&ioreq->blkdev->xendev, 0,
>> +                      "failed to copy data %d\n", rc);
>> +        ioreq->aio_errors++;
>> +        return -1;
>> +    }
>> +
>> +    for (i = 0; i < count; i++) {
>> +        if (segs[i].status != GNTST_okay) {
>> +            xen_be_printf(&ioreq->blkdev->xendev, 3,
>> +                          "failed to copy data %d for gref %d, domid %d\n",
>> +                          segs[i].status, ioreq->refs[i], ioreq->domids[i]);
>> +            ioreq->aio_errors++;
>> +            rc = -1;
>> +        }
>> +    }
>> +
>> +    return rc;
>> +}
>> +#else
>> +static void free_buffers(struct ioreq *ioreq)
>> +{
>> +    abort();
>> +}
>> +
>> +static int ioreq_init_copy_buffers(struct ioreq *ioreq)
>> +{
>> +    abort();
>> +}
>> +
>> +static int ioreq_copy(struct ioreq *ioreq)
>> +{
>> +    abort();
>> +}
>> +#endif
>> +
>>   static int ioreq_runio_qemu_aio(struct ioreq *ioreq);
>>
>>   static void qemu_aio_complete(void *opaque, int ret)
>> @@ -511,8 +616,31 @@ static void qemu_aio_complete(void *opaque, int ret)
>>           return;
>>       }
>>
>> +    if (ioreq->blkdev->feature_grant_copy) {
>> +        switch (ioreq->req.operation) {
>> +        case BLKIF_OP_READ:
>> +            /* in case of failure ioreq->aio_errors is increased */
>> +            if (ret == 0) {
>> +                ioreq_copy(ioreq);
>> +            }
>> +            free_buffers(ioreq);
>> +            break;
>> +        case BLKIF_OP_WRITE:
>> +        case BLKIF_OP_FLUSH_DISKCACHE:
>> +            if (!ioreq->req.nr_segments) {
>> +                break;
>> +            }
>> +            free_buffers(ioreq);
>> +            break;
>> +        default:
>> +            break;
>> +        }
>> +    }
>> +
>>       ioreq->status = ioreq->aio_errors ? BLKIF_RSP_ERROR : BLKIF_RSP_OKAY;
>> -    ioreq_unmap(ioreq);
>> +    if (!ioreq->blkdev->feature_grant_copy) {
>> +        ioreq_unmap(ioreq);
>> +    }
>>       ioreq_finish(ioreq);
>>       switch (ioreq->req.operation) {
>>       case BLKIF_OP_WRITE:
>> @@ -538,8 +666,20 @@ static int ioreq_runio_qemu_aio(struct ioreq *ioreq)
>>   {
>>       struct XenBlkDev *blkdev = ioreq->blkdev;
>>
>> -    if (ioreq->req.nr_segments && ioreq_map(ioreq) == -1) {
>> -        goto err_no_map;
>> +    if (ioreq->blkdev->feature_grant_copy) {
>> +        ioreq_init_copy_buffers(ioreq);
>> +        if (ioreq->req.nr_segments && (ioreq->req.operation == BLKIF_OP_WRITE ||
>> +            ioreq->req.operation == BLKIF_OP_FLUSH_DISKCACHE)) {
>> +            if (ioreq_copy(ioreq)) {
>> +                free_buffers(ioreq);
>> +                goto err;
>> +            }
>> +        }
>> +
>
> Please remove blank this line (it looks a bit odd)
>
>
>> +    } else {
>> +        if (ioreq->req.nr_segments && ioreq_map(ioreq)) {
>> +            goto err;
>> +        }
>>       }
>>
>>       ioreq->aio_inflight++;
>> @@ -582,6 +722,9 @@ static int ioreq_runio_qemu_aio(struct ioreq *ioreq)
>>       }
>>       default:
>>           /* unknown operation (shouldn't happen -- parse catches this) */
>> +        if (!ioreq->blkdev->feature_grant_copy) {
>> +            ioreq_unmap(ioreq);
>> +        }
>
> Why are you adding this? Is it a bug fix? If so, please explain in the
> commit message.
>
It is not a bug fix. In the ioreq_runio_qemu_aio there were two error 
labels 'err_no_map' and 'err' each of them used once. So before the 
patch the labels are looking this way:
err:
     ioreq_unmap(ioreq);
err_no_map:
     ioreq_finish(ioreq);
     ioreq->status = BLKIF_RSP_ERROR;
     return -1;

I removed the 'err_no_map' label and from the 'err' section I removed 
the ioreq_unmap. The 'err' label was previously used in that default 
section of the switch because the grant_map is called regardless the 
ioreq->req.operation.

In the patch, there is no need for any special behavior for grant_copy, 
because buffers were not allocated in this case, so there is only jump 
to 'err'.

An advantage of this change is one unified error path for both grant 
operations.
>
>>           goto err;
>>       }
>>
>> @@ -590,8 +733,6 @@ static int ioreq_runio_qemu_aio(struct ioreq *ioreq)
>>       return 0;
>>
>>   err:
>> -    ioreq_unmap(ioreq);
>> -err_no_map:
>>       ioreq_finish(ioreq);
>>       ioreq->status = BLKIF_RSP_ERROR;
>>       return -1;
>> @@ -1032,6 +1173,12 @@ static int blk_connect(struct XenDevice *xendev)
>>
>>       xen_be_bind_evtchn(&blkdev->xendev);
>>
>> +    blkdev->feature_grant_copy =
>> +                (xengnttab_grant_copy(blkdev->xendev.gnttabdev, 0, NULL) == 0);
>> +
>> +    xen_be_printf(&blkdev->xendev, 3, "grant copy operation %s\n",
>> +                  blkdev->feature_grant_copy ? "enabled" : "disabled");
>> +
>>       xen_be_printf(&blkdev->xendev, 1, "ok: proto %s, ring-ref %d, "
>>                     "remote port %d, local port %d\n",
>>                     blkdev->xendev.protocol, blkdev->ring_ref,
>> diff --git a/include/hw/xen/xen_common.h b/include/hw/xen/xen_common.h
>> index bd39287..8e1580d 100644
>> --- a/include/hw/xen/xen_common.h
>> +++ b/include/hw/xen/xen_common.h
>> @@ -424,4 +424,18 @@ static inline int xen_domain_create(xc_interface *xc, uint32_t ssidref,
>>   #endif
>>   #endif
>>
>> +/* Xen before 4.8 */
>> +
>> +#if CONFIG_XEN_CTRL_INTERFACE_VERSION < 480
>> +
>> +
>> +typedef void *xengnttab_grant_copy_segment_t;
>> +
>> +static inline int xengnttab_grant_copy(xengnttab_handle *xgt, uint32_t count,
>> +                                       xengnttab_grant_copy_segment_t *segs)
>> +{
>> +    return -ENOSYS;
>> +}
>> +#endif
>> +
>>   #endif /* QEMU_HW_XEN_COMMON_H */
>> --
>> 1.9.1
>>

Paulina