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Message-ID: <20230331162456.3f52b9e3.alex.williamson@redhat.com>
Date: Fri, 31 Mar 2023 16:24:56 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: <jgg@...dia.com>, <yishaih@...dia.com>,
<shameerali.kolothum.thodi@...wei.com>, <kevin.tian@...el.com>,
<tglx@...utronix.de>, <darwi@...utronix.de>, <kvm@...r.kernel.org>,
<dave.jiang@...el.com>, <jing2.liu@...el.com>,
<ashok.raj@...el.com>, <fenghua.yu@...el.com>,
<tom.zanussi@...ux.intel.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 7/8] vfio/pci: Support dynamic MSI-x
On Fri, 31 Mar 2023 10:49:16 -0700
Reinette Chatre <reinette.chatre@...el.com> wrote:
> Hi Alex,
>
> On 3/30/2023 3:42 PM, Alex Williamson wrote:
> > On Thu, 30 Mar 2023 16:40:50 -0600
> > Alex Williamson <alex.williamson@...hat.com> wrote:
> >
> >> On Tue, 28 Mar 2023 14:53:34 -0700
> >> Reinette Chatre <reinette.chatre@...el.com> wrote:
> >>
>
> ...
>
> >>> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> >>> index b3a258e58625..755b752ca17e 100644
> >>> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> >>> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> >>> @@ -55,6 +55,13 @@ struct vfio_pci_irq_ctx *vfio_irq_ctx_get(struct vfio_pci_core_device *vdev,
> >>> return xa_load(&vdev->ctx, index);
> >>> }
> >>>
> >>> +static void vfio_irq_ctx_free(struct vfio_pci_core_device *vdev,
> >>> + struct vfio_pci_irq_ctx *ctx, unsigned long index)
> >>> +{
> >>> + xa_erase(&vdev->ctx, index);
> >>> + kfree(ctx);
> >>> +}
> >
> > Also, the function below should use this rather than open coding the
> > same now. Thanks,
>
> It should, yes. Thank you. Will do.
>
>
> >>> static void vfio_irq_ctx_free_all(struct vfio_pci_core_device *vdev)
> >>> {
> >>> struct vfio_pci_irq_ctx *ctx;
> >>> @@ -409,33 +416,62 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
> >>> {
> >>> struct pci_dev *pdev = vdev->pdev;
> >>> struct vfio_pci_irq_ctx *ctx;
> >>> + struct msi_map msix_map = {};
> >>> + bool allow_dyn_alloc = false;
> >>> struct eventfd_ctx *trigger;
> >>> + bool new_ctx = false;
> >>> int irq, ret;
> >>> u16 cmd;
> >>>
> >>> + /* Only MSI-X allows dynamic allocation. */
> >>> + if (msix && pci_msix_can_alloc_dyn(vdev->pdev))
> >>> + allow_dyn_alloc = true;
> >>
> >> Should vfio-pci-core probe this and store it in a field on
> >> vfio_pci_core_device so that we can simply use something like
> >> vdev->has_dyn_msix throughout?
>
> It is not obvious to me if you mean this with vfio-pci-core probe,
> but it looks like a change to vfio_pci_core_enable() may be
> appropriate with a snippet like below:
>
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index a743b98ba29a..a474ce80a555 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -533,6 +533,8 @@ int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
> } else
> vdev->msix_bar = 0xFF;
>
> + vdev->has_dyn_msix = pci_msix_can_alloc_dyn(pdev);
> +
> if (!vfio_vga_disabled() && vfio_pci_is_vga(pdev))
> vdev->has_vga = true;
>
> Please do note that I placed it outside of the earlier "if (msix_pos)" since
> pci_msix_can_alloc_dyn() has its own "if (!dev->msix_cap)". If you prefer
> to keep all the vdev->*msix* together I can move it into the if statement.
Sure, just for common grouping I'd probably put it within the existing
msix_cap branch.
> With vdev->has_dyn_msix available "allow_dyn_alloc" can be dropped as you
> stated.
>
> >>
> >>> +
> >>> ctx = vfio_irq_ctx_get(vdev, vector);
> >>> - if (!ctx)
> >>> + if (!ctx && !allow_dyn_alloc)
> >>> return -EINVAL;
> >>> +
> >>> irq = pci_irq_vector(pdev, vector);
> >>> + /* Context and interrupt are always allocated together. */
> >>> + WARN_ON((ctx && irq == -EINVAL) || (!ctx && irq != -EINVAL));
> >>>
> >>> - if (ctx->trigger) {
> >>> + if (ctx && ctx->trigger) {
> >>> irq_bypass_unregister_producer(&ctx->producer);
> >>>
> >>> cmd = vfio_pci_memory_lock_and_enable(vdev);
> >>> free_irq(irq, ctx->trigger);
> >>> + if (allow_dyn_alloc) {
> >>
> >> It almost seems easier to define msix_map in each scope that it's used:
> >>
> >> struct msi_map map = { .index = vector,
> >> .virq = irq };
> >>
>
> Sure. Will do.
>
> >>> + msix_map.index = vector;
> >>> + msix_map.virq = irq;
> >>> + pci_msix_free_irq(pdev, msix_map);
> >>> + irq = -EINVAL;
> >>> + }
> >>> vfio_pci_memory_unlock_and_restore(vdev, cmd);
> >>> kfree(ctx->name);
> >>> eventfd_ctx_put(ctx->trigger);
> >>> ctx->trigger = NULL;
> >>> + if (allow_dyn_alloc) {
> >>> + vfio_irq_ctx_free(vdev, ctx, vector);
> >>> + ctx = NULL;
> >>> + }
> >>> }
> >>>
> >>> if (fd < 0)
> >>> return 0;
> >>>
> >>> + if (!ctx) {
> >>> + ctx = vfio_irq_ctx_alloc_single(vdev, vector);
> >>> + if (!ctx)
> >>> + return -ENOMEM;
> >>> + new_ctx = true;
> >>> + }
> >>> +
> >>> ctx->name = kasprintf(GFP_KERNEL_ACCOUNT, "vfio-msi%s[%d](%s)",
> >>> msix ? "x" : "", vector, pci_name(pdev));
> >>> - if (!ctx->name)
> >>> - return -ENOMEM;
> >>> + if (!ctx->name) {
> >>> + ret = -ENOMEM;
> >>> + goto out_free_ctx;
> >>> + }
> >>>
> >>> trigger = eventfd_ctx_fdget(fd);
> >>> if (IS_ERR(trigger)) {
> >>> @@ -443,25 +479,38 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
> >>> goto out_free_name;
> >>> }
> >>>
> >>> - /*
> >>> - * The MSIx vector table resides in device memory which may be cleared
> >>> - * via backdoor resets. We don't allow direct access to the vector
> >>> - * table so even if a userspace driver attempts to save/restore around
> >>> - * such a reset it would be unsuccessful. To avoid this, restore the
> >>> - * cached value of the message prior to enabling.
> >>> - */
> >>> cmd = vfio_pci_memory_lock_and_enable(vdev);
> >>> if (msix) {
> >>> - struct msi_msg msg;
> >>> -
> >>> - get_cached_msi_msg(irq, &msg);
> >>> - pci_write_msi_msg(irq, &msg);
> >>> + if (irq == -EINVAL) {
> >>> + msix_map = pci_msix_alloc_irq_at(pdev, vector, NULL);
> >>
> >> struct msi_map map = pci_msix_alloc_irq_at(pdev,
> >> vector, NULL);
>
> Will do.
>
> >>> + if (msix_map.index < 0) {
> >>> + vfio_pci_memory_unlock_and_restore(vdev, cmd);
> >>> + ret = msix_map.index;
> >>> + goto out_put_eventfd_ctx;
> >>> + }
> >>> + irq = msix_map.virq;
> >>> + } else {
> >>> + /*
> >>> + * The MSIx vector table resides in device memory which
> >>> + * may be cleared via backdoor resets. We don't allow
> >>> + * direct access to the vector table so even if a
> >>> + * userspace driver attempts to save/restore around
> >>> + * such a reset it would be unsuccessful. To avoid
> >>> + * this, restore the cached value of the message prior
> >>> + * to enabling.
> >>> + */
> >>
> >> You've only just copied this comment down to here, but I think it's a
> >> bit stale. Maybe we should update it to something that helps explain
> >> this split better, maybe:
> >>
> >> /*
> >> * If the vector was previously allocated, refresh the
> >> * on-device message data before enabling in case it had
> >> * been cleared or corrupted since writing.
> >> */
> >>
> >> IIRC, that was the purpose of writing it back to the device and the
> >> blocking of direct access is no longer accurate anyway.
>
> Thank you. Will do. To keep this patch focused I plan to separate
> this change into a new prep patch that will be placed earlier in
> this series.
Ok.
> >>
> >>> + struct msi_msg msg;
> >>> +
> >>> + get_cached_msi_msg(irq, &msg);
> >>> + pci_write_msi_msg(irq, &msg);
> >>> + }
> >>> }
> >>>
> >>> ret = request_irq(irq, vfio_msihandler, 0, ctx->name, trigger);
> >>> - vfio_pci_memory_unlock_and_restore(vdev, cmd);
> >>> if (ret)
> >>> - goto out_put_eventfd_ctx;
> >>> + goto out_free_irq_locked;
> >>> +
> >>> + vfio_pci_memory_unlock_and_restore(vdev, cmd);
> >>>
> >>> ctx->producer.token = trigger;
> >>> ctx->producer.irq = irq;
> >>> @@ -477,11 +526,21 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
> >>>
> >>> return 0;
> >>>
> >>> +out_free_irq_locked:
> >>> + if (allow_dyn_alloc && new_ctx) {
> >>
> >> struct msi_map map = { .index = vector,
> >> .virq = irq };
> >>
>
> Will do.
>
> >>> + msix_map.index = vector;
> >>> + msix_map.virq = irq;
> >>> + pci_msix_free_irq(pdev, msix_map);
> >>> + }
> >>> + vfio_pci_memory_unlock_and_restore(vdev, cmd);
> >>> out_put_eventfd_ctx:
> >>> eventfd_ctx_put(trigger);
> >>> out_free_name:
> >>> kfree(ctx->name);
> >>> ctx->name = NULL;
> >>> +out_free_ctx:
> >>> + if (allow_dyn_alloc && new_ctx)
> >>> + vfio_irq_ctx_free(vdev, ctx, vector);
> >>> return ret;
> >>> }
> >>>
> >>
> >> Do we really need the new_ctx test in the above cases? Thanks,
>
> new_ctx is not required for correctness but instead is used to keep
> the code symmetric.
> Specifically, if the user enables MSI-X without providing triggers and
> then later assign triggers then an error path without new_ctx would unwind
> more than done in this function, it would free the context that
> was allocated within vfio_msi_enable().
Seems like we already have that asymmetry, if a trigger is unset we'll
free the ctx allocated by vfio_msi_enable(). Tracking which are
allocated where is unnecessarily complex, how about a policy that
devices supporting vdev->has_dyn_msix only ever have active contexts
allocated? Thanks,
Alex
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