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

Oleksandr
2022-04-17T18:21:27Z

On 16.04.22 01:02, Stefano Stabellini wrote:

Hello Stefano

> 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

Yes, but please see my comments in commit #2 regarding
CONFIG_XEN_HVM_VIRTIO_GRANT option and
arch_has_restricted_virtio_memory_access() on Arm.

I propose to have the following on Arm:

int arch_has_restricted_virtio_memory_access(void)
{
return xen_has_restricted_virtio_memory_access();
}

where common xen.h contain a helper to be used by both Arm and x86:

static inline int xen_has_restricted_virtio_memory_access(void)
{
if (IS_ENABLED(CONFIG_XEN_VIRTIO) && (xen_pv_domain() ||
xen_hvm_domain()))
return 1;

return 0;
}

But I would be happy with what you propose as well.

>
>
>> + 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

According to the conversation in commit #2 we are considering just a
single XEN_VIRTIO option, but it is going to has the
same architectural dependencies: (X86_64 || ARM || ARM64)

By removing the architectural dependencies here, we will leave also
X86_32 covered (neither XEN_HVM_VIRTIO_GRANT nor XEN_PV_VIRTIO covered
it). I don't know whether it is ok or not.

Shall I remove dependencies anyway?

>
>
>> 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.

ok, will do

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

Really good question, I introduced a lock for other purpose, I was
thinking we needed to protect grants allocation and removing, but wasn't
100% sure about it (I wrote a remark in commit description). But looking
into grant_table.c again I see that grant table code uses it's own lock,
so looks like we don't need an extra lock here. I need to re-check
regarding "broken", but likely we don't need here as well. If so, I will
remove the lock.

>
>
>> +};
>> +
>> +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?

Good catch, thank you. Yes, it is impossible on Arm for now (with
changes in commit #6).

>
> 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;
> }

ok, will do

>
>
>
>> + 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 */

--
Regards,

Oleksandr Tyshchenko