Re: [Xen-devel] [PATCH v2] xen/HVM: atomically access pointers in bufioreq handling

[Fabio Fantoni <fabio.fantoni@xxxxxxx>]

Il 23/07/2015 08:59, Jan Beulich ha scritto:
> The number of slots per page being 511 (i.e. not a power of two) means
> that the (32-bit) read and write indexes going beyond 2^32 will likely
> disturb operation. The hypervisor side gets I/O req server creation
> extended so we can indicate that we're using suitable atomic accesses
> where needed, allowing it to atomically canonicalize both pointers when
> both have gone through at least one cycle.
>
> The Xen side counterpart (which is not a functional prereq to this
> change, albeit a build one) went in already (commit b7007bc6f9).
If I understand good a xen 4.6 is needed to build qemu with this patch, 
if yes probably is good add also a change to configure of require xen 
>=4.6, instead qemu will fail to build if xen version if lower.
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Adjust description.
>
> --- a/xen-hvm.c
> +++ b/xen-hvm.c
> @@ -957,19 +957,30 @@ static void handle_ioreq(XenIOState *sta
>   
>   static int handle_buffered_iopage(XenIOState *state)
>   {
> +    buffered_iopage_t *buf_page = state->buffered_io_page;
>       buf_ioreq_t *buf_req = NULL;
>       ioreq_t req;
>       int qw;
>   
> -    if (!state->buffered_io_page) {
> +    if (!buf_page) {
>           return 0;
>       }
>   
>       memset(&req, 0x00, sizeof(req));
>   
> -    while (state->buffered_io_page->read_pointer != state->buffered_io_page->write_pointer) {
> -        buf_req = &state->buffered_io_page->buf_ioreq[
> -            state->buffered_io_page->read_pointer % IOREQ_BUFFER_SLOT_NUM];
> +    for (;;) {
> +        uint32_t rdptr = buf_page->read_pointer, wrptr;
> +
> +        xen_rmb();
> +        wrptr = buf_page->write_pointer;
> +        xen_rmb();
> +        if (rdptr != buf_page->read_pointer) {
> +            continue;
> +        }
> +        if (rdptr == wrptr) {
> +            break;
> +        }
> +        buf_req = &buf_page->buf_ioreq[rdptr % IOREQ_BUFFER_SLOT_NUM];
>           req.size = 1UL << buf_req->size;
>           req.count = 1;
>           req.addr = buf_req->addr;
> @@ -981,15 +992,14 @@ static int handle_buffered_iopage(XenIOS
>           req.data_is_ptr = 0;
>           qw = (req.size == 8);
>           if (qw) {
> -            buf_req = &state->buffered_io_page->buf_ioreq[
> -                (state->buffered_io_page->read_pointer + 1) % 
> IOREQ_BUFFER_SLOT_NUM];
> +            buf_req = &buf_page->buf_ioreq[(rdptr + 1) %
> +                                           IOREQ_BUFFER_SLOT_NUM];
>               req.data |= ((uint64_t)buf_req->data) << 32;
>           }
>   
>           handle_ioreq(state, &req);
>   
> -        xen_mb();
> -        state->buffered_io_page->read_pointer += qw ? 2 : 1;
> +        atomic_add(&buf_page->read_pointer, qw + 1);
>       }
>   
>       return req.count;
> --- unstable.orig/qemu/upstream/include/hw/xen/xen_common.h     2015-03-31 
> 15:58:04.000000000 +0200
> +++ unstable/qemu/upstream/include/hw/xen/xen_common.h  2015-06-15 
> 08:24:28.000000000 +0200
> @@ -370,7 +370,8 @@ static inline void xen_unmap_pcidev(XenX
>   static inline int xen_create_ioreq_server(XenXC xc, domid_t dom,
>                                             ioservid_t *ioservid)
>   {
> -    int rc = xc_hvm_create_ioreq_server(xc, dom, 1, ioservid);
> +    int rc = xc_hvm_create_ioreq_server(xc, dom, HVM_IOREQSRV_BUFIOREQ_ATOMIC,
> +                                        ioservid);
>   
>       if (rc == 0) {
>           trace_xen_ioreq_server_create(*ioservid);
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