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Re: [Qemu-devel] [PATCH v3] xen/HVM: atomically access pointers in bufio
From: |
Stefano Stabellini |
Subject: |
Re: [Qemu-devel] [PATCH v3] xen/HVM: atomically access pointers in bufioreq handling |
Date: |
Fri, 24 Jul 2015 11:34:22 +0100 |
User-agent: |
Alpine 2.02 (DEB 1266 2009-07-14) |
On Fri, 24 Jul 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, 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).
>
> Signed-off-by: Jan Beulich <address@hidden>
I have queued it up for QEMU 2.5. I'll backport to Xen 4.6.
> v3: Check for Xen 4.6 (configure and build time).
> v2: Adjust description.
>
> --- a/configure
> +++ b/configure
> @@ -1882,6 +1882,33 @@ int main(void) {
> xc_gnttab_open(NULL, 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);
> + return 0;
> +}
> +EOF
> + compile_prog "" "$xen_libs"
> + then
> + xen_ctrl_version=460
> + xen=yes
> +
> + # Xen 4.5
> + elif
> + cat > $TMPC <<EOF &&
> +#include <xenctrl.h>
> +#include <xenstore.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;
> + xs_daemon_open();
> + xc = xc_interface_open(0, 0, 0);
> + xc_hvm_set_mem_type(0, 0, HVMMEM_ram_ro, 0, 0);
> + xc_gnttab_open(NULL, 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, 0, NULL);
> return 0;
> }
> --- a/xen-hvm.c
> +++ b/xen-hvm.c
> @@ -963,19 +963,30 @@ 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;
> @@ -987,15 +998,14 @@ 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;
> --- a/include/hw/xen/xen_common.h
> +++ b/include/hw/xen/xen_common.h
> @@ -186,6 +186,15 @@ static inline int xen_get_vmport_regs_pf
> }
> #endif
>
> +/* Xen before 4.6 */
> +#if CONFIG_XEN_CTRL_INTERFACE_VERSION < 460
> +
> +#ifndef HVM_IOREQSRV_BUFIOREQ_ATOMIC
> +#define HVM_IOREQSRV_BUFIOREQ_ATOMIC 2
> +#endif
> +
> +#endif
> +
> /* Xen before 4.5 */
> #if CONFIG_XEN_CTRL_INTERFACE_VERSION < 450
>
> @@ -370,7 +379,8 @@ 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);
>
>
>