Re: [RFC PATCH 4/6] virtio: Various updates to xen-virtio DMA ops layer

Stefano Stabellini
2022-04-15T22:02:30Z

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On Thu, 14 Apr 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>
> In the context of current patch do the following:
> 1. Update code to support virtio-mmio devices
> 2. Introduce struct xen_virtio_data and account passed virtio devices
> (using list) as we need to store some per-device data
> 3. Add multi-page support for xen_virtio_dma_map(unmap)_page callbacks
> 4. Harden code against malicious backend
> 5. Change to use alloc_pages_exact() instead of __get_free_pages()
> 6. Introduce locking scheme to protect mappings (I am not 100% sure
> whether per-device lock is really needed)
> 7. Handle virtio device's DMA mask
> 8. Retrieve the ID of backend domain from DT for virtio-mmio device
> instead of hardcoding it.
>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> ---
> arch/arm/xen/enlighten.c | 11 +++
> drivers/xen/Kconfig | 2 +-
> drivers/xen/xen-virtio.c | 200 ++++++++++++++++++++++++++++++++++++++++++-----
> include/xen/xen-ops.h | 5 ++
> 4 files changed, 196 insertions(+), 22 deletions(-)
>
> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> index ec5b082..870d92f 100644
> --- a/arch/arm/xen/enlighten.c
> +++ b/arch/arm/xen/enlighten.c
> @@ -409,6 +409,17 @@ int __init arch_xen_unpopulated_init(struct resource **res)
> }
> #endif
>
> +#ifdef CONFIG_ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
> +int arch_has_restricted_virtio_memory_access(void)
> +{
> + if (IS_ENABLED(CONFIG_XEN_HVM_VIRTIO_GRANT) && xen_hvm_domain())
> + return 1;

Instead of xen_hvm_domain(), you can just use xen_domain(). Also there
is no need for the #ifdef
CONFIG_ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS, given that:

CONFIG_XEN_HVM_VIRTIO_GRANT depends on XEN_VIRTIO which selects
ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS

> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
> +#endif
> +
> static void __init xen_dt_guest_init(void)
> {
> struct device_node *xen_node;
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index fc61f7a..56afe6a 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -347,7 +347,7 @@ config XEN_VIRTIO
>
> config XEN_HVM_VIRTIO_GRANT
> bool "Require virtio for fully virtualized guests to use grant mappings"
> - depends on XEN_VIRTIO && X86_64
> + depends on XEN_VIRTIO && (X86_64 || ARM || ARM64)

you can remove the architectural dependencies

> default y
> help
> Require virtio for fully virtualized guests to use grant mappings.
> diff --git a/drivers/xen/xen-virtio.c b/drivers/xen/xen-virtio.c
> index cfd5eda..c5b2ec9 100644
> --- a/drivers/xen/xen-virtio.c
> +++ b/drivers/xen/xen-virtio.c
> @@ -7,12 +7,26 @@
>
> #include <linux/module.h>
> #include <linux/dma-map-ops.h>
> +#include <linux/of.h>
> #include <linux/pci.h>
> #include <linux/pfn.h>
> #include <linux/virtio_config.h>
> #include <xen/xen.h>
> #include <xen/grant_table.h>
>
> +struct xen_virtio_data {
> + /* The ID of backend domain */
> + domid_t dev_domid;
> + struct device *dev;
> + struct list_head list;
> + spinlock_t lock;
> + /* Is device behaving sane? */
> + bool broken;

If you moved "broken" after "dev_domid" we would save a few bytes for
every allocation due to padding.

Is data->lock only there to protect accesses to "broken"? If so, we
might not need it, but I am not sure.

> +};
> +
> +static LIST_HEAD(xen_virtio_devices);
> +static DEFINE_SPINLOCK(xen_virtio_lock);
> +
> #define XEN_GRANT_ADDR_OFF 0x8000000000000000ULL
>
> static inline dma_addr_t grant_to_dma(grant_ref_t grant)
> @@ -25,6 +39,25 @@ static inline grant_ref_t dma_to_grant(dma_addr_t dma)
> return (grant_ref_t)((dma & ~XEN_GRANT_ADDR_OFF) >> PAGE_SHIFT);
> }
>
> +static struct xen_virtio_data *find_xen_virtio_data(struct device *dev)
> +{
> + struct xen_virtio_data *data = NULL;
> + bool found = false;
> +
> + spin_lock(&xen_virtio_lock);
> +
> + list_for_each_entry( data, &xen_virtio_devices, list) {
> + if (data->dev == dev) {
> + found = true;
> + break;
> + }
> + }
> +
> + spin_unlock(&xen_virtio_lock);
> +
> + return found ? data : NULL;
> +}
> +
> /*
> * DMA ops for Xen virtio frontends.
> *
> @@ -43,48 +76,78 @@ static void *xen_virtio_dma_alloc(struct device *dev, size_t size,
> dma_addr_t *dma_handle, gfp_t gfp,
> unsigned long attrs)
> {
> - unsigned int n_pages = PFN_UP(size);
> - unsigned int i;
> + struct xen_virtio_data *data;
> + unsigned int i, n_pages = PFN_UP(size);
> unsigned long pfn;
> grant_ref_t grant;
> - void *ret;
> + void *ret = NULL;
>
> - ret = (void *)__get_free_pages(gfp, get_order(size));
> - if (!ret)
> + data = find_xen_virtio_data(dev);
> + if (!data)
> return NULL;
>
> + spin_lock(&data->lock);
> +
> + if (unlikely(data->broken))
> + goto out;
> +
> + ret = alloc_pages_exact(n_pages * PAGE_SIZE, gfp);
> + if (!ret)
> + goto out;
> +
> pfn = virt_to_pfn(ret);
>
> if (gnttab_alloc_grant_reference_seq(n_pages, &grant)) {
> - free_pages((unsigned long)ret, get_order(size));
> - return NULL;
> + free_pages_exact(ret, n_pages * PAGE_SIZE);
> + ret = NULL;
> + goto out;
> }
>
> for (i = 0; i < n_pages; i++) {
> - gnttab_grant_foreign_access_ref(grant + i, 0,
> + gnttab_grant_foreign_access_ref(grant + i, data->dev_domid,
> pfn_to_gfn(pfn + i), 0);
> }
>
> *dma_handle = grant_to_dma(grant);
>
> +out:
> + spin_unlock(&data->lock);
> +
> return ret;
> }
>
> static void xen_virtio_dma_free(struct device *dev, size_t size, void *vaddr,
> dma_addr_t dma_handle, unsigned long attrs)
> {
> - unsigned int n_pages = PFN_UP(size);
> - unsigned int i;
> + struct xen_virtio_data *data;
> + unsigned int i, n_pages = PFN_UP(size);
> grant_ref_t grant;
>
> + data = find_xen_virtio_data(dev);
> + if (!data)
> + return;
> +
> + spin_lock(&data->lock);
> +
> + if (unlikely(data->broken))
> + goto out;
> +
> grant = dma_to_grant(dma_handle);
>
> - for (i = 0; i < n_pages; i++)
> - gnttab_end_foreign_access_ref(grant + i);
> + for (i = 0; i < n_pages; i++) {
> + if (unlikely(!gnttab_end_foreign_access_ref(grant + i))) {
> + dev_alert(dev, "Grant still in use by backend domain, disabled for further use\n");
> + data->broken = true;
> + goto out;
> + }
> + }
>
> gnttab_free_grant_reference_seq(grant, n_pages);
>
> - free_pages((unsigned long)vaddr, get_order(size));
> + free_pages_exact(vaddr, n_pages * PAGE_SIZE);
> +
> +out:
> + spin_unlock(&data->lock);
> }
>
> static struct page *xen_virtio_dma_alloc_pages(struct device *dev, size_t size,
> @@ -108,28 +171,71 @@ static dma_addr_t xen_virtio_dma_map_page(struct device *dev, struct page *page,
> enum dma_data_direction dir,
> unsigned long attrs)
> {
> + struct xen_virtio_data *data;
> + unsigned int i, n_pages = PFN_UP(size);
> grant_ref_t grant;
> + dma_addr_t dma_handle = DMA_MAPPING_ERROR;
> +
> + BUG_ON(dir == DMA_NONE);
> +
> + data = find_xen_virtio_data(dev);
> + if (!data)
> + return DMA_MAPPING_ERROR;
> +
> + spin_lock(&data->lock);
>
> - if (gnttab_alloc_grant_references(1, &grant))
> - return 0;
> + if (unlikely(data->broken))
> + goto out;
>
> - gnttab_grant_foreign_access_ref(grant, 0, xen_page_to_gfn(page),
> - dir == DMA_TO_DEVICE);
> + if (gnttab_alloc_grant_reference_seq(n_pages, &grant))
> + goto out;
>
> - return grant_to_dma(grant) + offset;
> + for (i = 0; i < n_pages; i++) {
> + gnttab_grant_foreign_access_ref(grant + i, data->dev_domid,
> + xen_page_to_gfn(page) + i, dir == DMA_TO_DEVICE);
> + }
> +
> + dma_handle = grant_to_dma(grant) + offset;
> +
> +out:
> + spin_unlock(&data->lock);
> +
> + return dma_handle;
> }
>
> static void xen_virtio_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
> size_t size, enum dma_data_direction dir,
> unsigned long attrs)
> {
> + struct xen_virtio_data *data;
> + unsigned int i, n_pages = PFN_UP(size);
> grant_ref_t grant;
>
> + BUG_ON(dir == DMA_NONE);
> +
> + data = find_xen_virtio_data(dev);
> + if (!data)
> + return;
> +
> + spin_lock(&data->lock);
> +
> + if (unlikely(data->broken))
> + goto out;
> +
> grant = dma_to_grant(dma_handle);
>
> - gnttab_end_foreign_access_ref(grant);
> + for (i = 0; i < n_pages; i++) {
> + if (unlikely(!gnttab_end_foreign_access_ref(grant + i))) {
> + dev_alert(dev, "Grant still in use by backend domain, disabled for further use\n");
> + data->broken = true;
> + goto out;
> + }
> + }
> +
> + gnttab_free_grant_reference_seq(grant, n_pages);
>
> - gnttab_free_grant_reference(grant);
> +out:
> + spin_unlock(&data->lock);
> }
>
> static int xen_virtio_dma_map_sg(struct device *dev, struct scatterlist *sg,
> @@ -149,7 +255,7 @@ static void xen_virtio_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
>
> static int xen_virtio_dma_dma_supported(struct device *dev, u64 mask)
> {
> - return 1;
> + return mask == DMA_BIT_MASK(64);
> }
>
> static const struct dma_map_ops xen_virtio_dma_ops = {
> @@ -166,9 +272,61 @@ static const struct dma_map_ops xen_virtio_dma_ops = {
> .dma_supported = xen_virtio_dma_dma_supported,
> };
>
> +bool xen_is_virtio_device(struct device *dev)
> +{
> + /* XXX Handle only DT devices for now */
> + if (!dev->of_node)
> + return false;
> +
> + if (!of_device_is_compatible(dev->of_node, "virtio,mmio"))
> + return false;
> +
> + return of_property_read_bool(dev->of_node, "xen,dev-domid");
> +}
> +EXPORT_SYMBOL_GPL(xen_is_virtio_device);
> +
> void xen_virtio_setup_dma_ops(struct device *dev)
> {
> + struct xen_virtio_data *data;
> + uint32_t dev_domid;
> +
> + data = find_xen_virtio_data(dev);
> + if (data) {
> + dev_err(dev, "xen_virtio data is already created\n");
> + return;
> + }
> +
> + if (dev_is_pci(dev)) {
> + /* XXX Leave it hard wired to dom0 for now */
> + dev_domid = 0;
> + } else if (dev->of_node) {
> + if (of_property_read_u32(dev->of_node, "xen,dev-domid", &dev_domid)) {
> + dev_err(dev, "xen,dev-domid property is not present\n");
> + goto err;
> + }
> + } else
> + /* The ACPI case is not supported */
> + goto err;

If we get here, it means that xen_is_virtio_device returned true, so the
PCI case is actually impossible?

I would rewrite these checks like this:

/* XXX: ACPI and PCI unsupported for now */
if (dev_is_pci(dev) || !dev->of_node) {
goto err;
}
if (of_property_read_u32(dev->of_node, "xen,dev-domid", &dev_domid)) {
dev_err(dev, "xen,dev-domid property is not present\n");
goto err;
}

> + data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
> + if (!data) {
> + dev_err(dev, "Сannot allocate xen_virtio data\n");
> + goto err;
> + }
> + data->dev_domid = dev_domid;
> + data->dev = dev;
> + spin_lock_init(&data->lock);
> +
> + spin_lock(&xen_virtio_lock);
> + list_add(&data->list, &xen_virtio_devices);
> + spin_unlock(&xen_virtio_lock);
> +
> dev->dma_ops = &xen_virtio_dma_ops;
> +
> + return;
> +
> +err:
> + dev_err(dev, "Сannot set up xen_virtio DMA ops, retain platform DMA ops\n");
> }
> EXPORT_SYMBOL_GPL(xen_virtio_setup_dma_ops);
>
> diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
> index ae3c1bc..fdbcb99 100644
> --- a/include/xen/xen-ops.h
> +++ b/include/xen/xen-ops.h
> @@ -223,10 +223,15 @@ static inline void xen_preemptible_hcall_end(void) { }
>
> #ifdef CONFIG_XEN_VIRTIO
> void xen_virtio_setup_dma_ops(struct device *dev);
> +bool xen_is_virtio_device(struct device *dev);
> #else
> static inline void xen_virtio_setup_dma_ops(struct device *dev)
> {
> }
> +static inline bool xen_is_virtio_device(struct device *dev)
> +{
> + return false;
> +}
> #endif /* CONFIG_XEN_VIRTIO */
>
> #endif /* INCLUDE_XEN_OPS_H */