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SubjectRe: [PATCH] Xen backend support for paged out grant targets.
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On Sep 12, 2012, at 4:21 PM, Andres Lagar-Cavilla wrote:

>
> On Sep 12, 2012, at 4:06 PM, Konrad Rzeszutek Wilk wrote:
>
>> On Wed, Sep 12, 2012 at 03:45:53PM -0400, Andres Lagar-Cavilla wrote:
>>> Since Xen-4.2, hvm domains may have portions of their memory paged out. When a
>>> foreign domain (such as dom0) attempts to map these frames, the map will
>>> initially fail. The hypervisor returns a suitable errno, and kicks an
>>> asynchronous page-in operation carried out by a helper. The foreign domain is
>>> expected to retry the mapping operation until it eventually succeeds. The
>>> foreign domain is not put to sleep because itself could be the one running the
>>> pager assist (typical scenario for dom0).
>>
>> Looks ok to me. You forgot to put on the CC LKML and netdev mailing list
>> so I did that for you.
> Thanks. I have a concern though (re-posting from xen-devel). I need to bring attention to the following hunk:
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 682633b..5610fd8 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -635,9 +635,7 @@ static void xen_netbk_rx_action(struct xen_netbk *netbk)
> return;
>
> BUG_ON(npo.copy_prod > ARRAY_SIZE(netbk->grant_copy_op));
> - ret = HYPERVISOR_grant_table_op(GNTTABOP_copy, &netbk->grant_copy_op,
> - npo.copy_prod);
> - BUG_ON(ret != 0);
> + gnttab_batch_copy_no_eagain(netbk->grant_copy_op, npo.copy_prod);
>
> It seems like the (extant) code passes a struct gnttab_copy ** to the grant table hypercall (&netbk->grant_copy_op, defined as struct gnttab_copy [])
>
> I "fixed" it to pass a struct gnttab_copy * (netbk->grant_copy_op, no '&'). This matches the other invocation of the grant copy hyper call (in netbk_tx_action). Did I fix it, though? I am confused as to how this could have ever worked.

Well, a bit of a faceslap moment for me.

netbk->grant_copy_op and &netbk->grant_copy_op yield the same pointer, thanks to obscure rule number foo:
"""Under ANSI/ISO Standard C, &array yields a pointer, of type pointer-to-array-of-T, to the entire array. Under pre-ANSI C, the & in &array generally elicited a warning, and was generally ignored. Under all C compilers, an unadorned reference to an array yields a pointer, of type pointer-to-T, to the array's first element. """

Please review patch as is.

Thanks and pardon the noise.
Andres

>
> Andres
>
>>
>>>
>>> This patch adds support for this mechanism for backend drivers using grant
>>> mapping and copying operations. Specifically, this covers the blkback and
>>> gntdev drivers (which map foregin grants), and the netback driver (which copies
>>> foreign grants).
>>>
>>> * Add GNTST_eagain, already exposed by Xen, to the grant interface.
>>> * Add a retry method for grants that fail with GNTST_eagain (i.e. because the
>>> target foregin frame is paged out).
>>> * Insert hooks with appropriate macro decorators in the aforementioned drivers.
>>>
>>> The retry loop is only invoked if the grant operation status is GNTST_eagain.
>>> It guarantees to leave a new status code different from GNTST_eagain. Any other
>>> status code results in identical code execution as before.
>>>
>>> The retry loop performs 256 attempts with increasing time intervals through a
>>> 32 second period. It uses msleep to yield while waiting for the next retry.
>>>
>>> V2 after feedback from David Vrabel:
>>> * Explicit MAX_DELAY instead of wrap-around delay into zero
>>> * Abstract GNTST_eagain check into core grant table code for netback module.
>>>
>>> Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
>>> ---
>>> drivers/net/xen-netback/netback.c | 11 +++-------
>>> drivers/xen/grant-table.c | 26 ++++++++++++++++++++++++
>>> drivers/xen/xenbus/xenbus_client.c | 6 ++----
>>> include/xen/grant_table.h | 38 +++++++++++++++++++++++++++++++++++
>>> include/xen/interface/grant_table.h | 2 ++
>>> 5 files changed, 71 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
>>> index 682633b..fc40258 100644
>>> --- a/drivers/net/xen-netback/netback.c
>>> +++ b/drivers/net/xen-netback/netback.c
>>> @@ -635,9 +635,7 @@ static void xen_netbk_rx_action(struct xen_netbk *netbk)
>>> return;
>>>
>>> BUG_ON(npo.copy_prod > ARRAY_SIZE(netbk->grant_copy_op));
>>> - ret = HYPERVISOR_grant_table_op(GNTTABOP_copy, &netbk->grant_copy_op,
>>> - npo.copy_prod);
>>> - BUG_ON(ret != 0);
>>> + gnttab_batch_copy_retry_eagain(netbk->grant_copy_op, npo.copy_prod);
>>>
>>> while ((skb = __skb_dequeue(&rxq)) != NULL) {
>>> sco = (struct skb_cb_overlay *)skb->cb;
>>> @@ -1460,18 +1458,15 @@ static void xen_netbk_tx_submit(struct xen_netbk *netbk)
>>> static void xen_netbk_tx_action(struct xen_netbk *netbk)
>>> {
>>> unsigned nr_gops;
>>> - int ret;
>>>
>>> nr_gops = xen_netbk_tx_build_gops(netbk);
>>>
>>> if (nr_gops == 0)
>>> return;
>>> - ret = HYPERVISOR_grant_table_op(GNTTABOP_copy,
>>> - netbk->tx_copy_ops, nr_gops);
>>> - BUG_ON(ret);
>>>
>>> - xen_netbk_tx_submit(netbk);
>>> + gnttab_batch_copy_retry_eagain(netbk->tx_copy_ops, nr_gops);
>>>
>>> + xen_netbk_tx_submit(netbk);
>>> }
>>>
>>> static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx)
>>> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
>>> index eea81cf..96543b2 100644
>>> --- a/drivers/xen/grant-table.c
>>> +++ b/drivers/xen/grant-table.c
>>> @@ -38,6 +38,7 @@
>>> #include <linux/vmalloc.h>
>>> #include <linux/uaccess.h>
>>> #include <linux/io.h>
>>> +#include <linux/delay.h>
>>> #include <linux/hardirq.h>
>>>
>>> #include <xen/xen.h>
>>> @@ -823,6 +824,26 @@ unsigned int gnttab_max_grant_frames(void)
>>> }
>>> EXPORT_SYMBOL_GPL(gnttab_max_grant_frames);
>>>
>>> +#define MAX_DELAY 256
>>> +void
>>> +gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t *status,
>>> + const char *func)
>>> +{
>>> + unsigned delay = 1;
>>> +
>>> + do {
>>> + BUG_ON(HYPERVISOR_grant_table_op(cmd, gop, 1));
>>> + if (*status == GNTST_eagain)
>>> + msleep(delay++);
>>> + } while ((*status == GNTST_eagain) && (delay < MAX_DELAY));
>>> +
>>> + if (delay >= MAX_DELAY) {
>>> + printk(KERN_ERR "%s: %s eagain grant\n", func, current->comm);
>>> + *status = GNTST_bad_page;
>>> + }
>>> +}
>>> +EXPORT_SYMBOL_GPL(gnttab_retry_eagain_gop);
>>> +
>>> int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>>> struct gnttab_map_grant_ref *kmap_ops,
>>> struct page **pages, unsigned int count)
>>> @@ -836,6 +857,11 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>>> if (ret)
>>> return ret;
>>>
>>> + /* Retry eagain maps */
>>> + for (i = 0; i < count; i++)
>>> + if (map_ops[i].status == GNTST_eagain)
>>> + gnttab_retry_eagain_map(map_ops + i);
>>> +
>>> if (xen_feature(XENFEAT_auto_translated_physmap))
>>> return ret;
>>>
>>> diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c
>>> index b3e146e..a6e07ea 100644
>>> --- a/drivers/xen/xenbus/xenbus_client.c
>>> +++ b/drivers/xen/xenbus/xenbus_client.c
>>> @@ -490,8 +490,7 @@ static int xenbus_map_ring_valloc_pv(struct xenbus_device *dev,
>>>
>>> op.host_addr = arbitrary_virt_to_machine(pte).maddr;
>>>
>>> - if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
>>> - BUG();
>>> + gnttab_map_grant_retry_eagain(&op);
>>>
>>> if (op.status != GNTST_okay) {
>>> free_vm_area(area);
>>> @@ -572,8 +571,7 @@ int xenbus_map_ring(struct xenbus_device *dev, int gnt_ref,
>>> gnttab_set_map_op(&op, (unsigned long)vaddr, GNTMAP_host_map, gnt_ref,
>>> dev->otherend_id);
>>>
>>> - if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
>>> - BUG();
>>> + gnttab_map_grant_retry_eagain(&op);
>>>
>>> if (op.status != GNTST_okay) {
>>> xenbus_dev_fatal(dev, op.status,
>>> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
>>> index 11e27c3..c829c83 100644
>>> --- a/include/xen/grant_table.h
>>> +++ b/include/xen/grant_table.h
>>> @@ -43,6 +43,7 @@
>>> #include <xen/interface/grant_table.h>
>>>
>>> #include <asm/xen/hypervisor.h>
>>> +#include <asm/xen/hypercall.h>
>>>
>>> #include <xen/features.h>
>>>
>>> @@ -183,6 +184,43 @@ unsigned int gnttab_max_grant_frames(void);
>>>
>>> #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
>>>
>>> +/* Retry a grant map/copy operation when the hypervisor returns GNTST_eagain.
>>> + * This is typically due to paged out target frames.
>>> + * Generic entry-point, use macro decorators below for specific grant
>>> + * operations.
>>> + * Will retry for 1, 2, ... 255 ms, i.e. 256 times during 32 seconds.
>>> + * Return value in *status guaranteed to no longer be GNTST_eagain. */
>>> +void gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t *status,
>>> + const char *func);
>>> +
>>> +#define gnttab_retry_eagain_map(_gop) \
>>> + gnttab_retry_eagain_gop(GNTTABOP_map_grant_ref, (_gop), \
>>> + &(_gop)->status, __func__)
>>> +
>>> +#define gnttab_retry_eagain_copy(_gop) \
>>> + gnttab_retry_eagain_gop(GNTTABOP_copy, (_gop), \
>>> + &(_gop)->status, __func__)
>>> +
>>> +static inline void
>>> +gnttab_map_grant_retry_eagain(struct gnttab_map_grant_ref *op)
>>> +{
>>> + BUG_ON((HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, op, 1)));
>>> + if (op->status == GNTST_eagain)
>>> + gnttab_retry_eagain_map(op);
>>> +}
>>> +
>>> +static inline void
>>> +gnttab_batch_copy_retry_eagain(struct gnttab_copy *batch, unsigned count)
>>> +{
>>> + unsigned i;
>>> + struct gnttab_copy *op;
>>> +
>>> + BUG_ON(HYPERVISOR_grant_table_op(GNTTABOP_copy, batch, count));
>>> + for (i = 0, op = batch; i < count; i++, op++)
>>> + if (op->status == GNTST_eagain)
>>> + gnttab_retry_eagain_copy(op);
>>> +}
>>> +
>>> int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>>> struct gnttab_map_grant_ref *kmap_ops,
>>> struct page **pages, unsigned int count);
>>> diff --git a/include/xen/interface/grant_table.h b/include/xen/interface/grant_table.h
>>> index 7da811b..66cb734 100644
>>> --- a/include/xen/interface/grant_table.h
>>> +++ b/include/xen/interface/grant_table.h
>>> @@ -520,6 +520,7 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
>>> #define GNTST_permission_denied (-8) /* Not enough privilege for operation. */
>>> #define GNTST_bad_page (-9) /* Specified page was invalid for op. */
>>> #define GNTST_bad_copy_arg (-10) /* copy arguments cross page boundary */
>>> +#define GNTST_eagain (-12) /* Retry. */
>>>
>>> #define GNTTABOP_error_msgs { \
>>> "okay", \
>>> @@ -533,6 +534,7 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
>>> "permission denied", \
>>> "bad page", \
>>> "copy arguments cross page boundary" \
>>> + "retry" \
>>> }
>>>
>>> #endif /* __XEN_PUBLIC_GRANT_TABLE_H__ */
>>> --
>>> 1.7.9.5
>> --
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