From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
qemu-devel@nongnu.org,
Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Subject: Re: [Qemu-devel] [PATCH] xen/HVM: atomically access pointers in bufioreq handling
Date: Tue, 21 Jul 2015 14:54:45 +0100 [thread overview]
Message-ID: <alpine.DEB.2.02.1507201723370.17378@kaball.uk.xensource.com> (raw)
In-Reply-To: <5582E14F0200007800086A37@mail.emea.novell.com>
On Thu, 18 Jun 2015, Jan Beulich wrote:
> 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 (not all accesses to the two pointers really need to be
> atomic), allowing it to atomically canonicalize both pointers when both
> have gone through at least one cycle.
The description is a bit terse: which accesses don't really need to be
atomic?
> The Xen side counterpart (which is not a functional prereq to this
> change, albeit a build one) can be found at e.g.
> http://lists.xenproject.org/archives/html/xen-devel/2015-06/msg02996.html
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> --- a/xen-hvm.c
> +++ b/xen-hvm.c
> @@ -981,19 +981,30 @@ 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();
We don't need this barrier.
> + wrptr = buf_page->write_pointer;
> + xen_rmb();
> + if (rdptr != buf_page->read_pointer) {
I think you have to use atomic_read to be sure that the second read to
buf_page->read_pointer is up to date and not optimized away.
But if I think that it would be best to simply use atomic_read to read
both pointers at once using uint64_t as type, so you are sure to get a
consistent view and there is no need for this check.
> + 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;
> @@ -1005,15 +1016,14 @@ 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);
I couldn't get specific info on the type of barrier implemented by
__sync_fetch_and_add, so I cannot tell for sure whether removing
xen_mb() is appropriate. Do you have a link? I suspect that given the
strong guarantees of the x86 architecture we'll be fine. I would be less
confident if this code was used on other archs.
> }
>
> return req.count;
> --- a/include/hw/xen/xen_common.h
> +++ b/include/hw/xen/xen_common.h
> @@ -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);
I am concerned that passing 2 instead of 1 could break older
hypervisors. However handle_bufioreq was never defined as a true
boolean, so maybe it is OK?
The alternative would be to create a xen_xc_hvm_create_ioreq_server
versioned wrapper in include/hw/xen/xen_common.h.
> if (rc == 0) {
> trace_xen_ioreq_server_create(*ioservid);
next prev parent reply other threads:[~2015-07-21 13:56 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-18 13:18 [Qemu-devel] [PATCH] xen/HVM: atomically access pointers in bufioreq handling Jan Beulich
2015-07-21 13:54 ` Stefano Stabellini
2015-07-21 13:54 ` Stefano Stabellini [this message]
2015-07-22 14:03 ` Jan Beulich
2015-07-22 14:03 ` [Qemu-devel] " Jan Beulich
2015-07-22 14:50 ` Stefano Stabellini
2015-07-22 15:34 ` Jan Beulich
2015-07-22 15:34 ` [Qemu-devel] " Jan Beulich
2015-07-22 17:24 ` Stefano Stabellini
2015-07-22 17:24 ` [Qemu-devel] " Stefano Stabellini
2015-07-22 17:26 ` Stefano Stabellini
2015-07-22 17:26 ` Stefano Stabellini
2015-07-23 7:02 ` [Qemu-devel] " Jan Beulich
2015-07-23 10:04 ` Stefano Stabellini
2015-07-23 10:04 ` [Qemu-devel] " Stefano Stabellini
2015-07-23 10:09 ` Stefano Stabellini
2015-07-23 10:09 ` [Qemu-devel] " Stefano Stabellini
2015-07-23 11:20 ` Jan Beulich
2015-07-23 11:20 ` Jan Beulich
2015-07-23 7:02 ` Jan Beulich
2015-07-22 14:50 ` Stefano Stabellini
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