Date:   Thu, 14 Apr 2022 12:43:33 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
CC:     Juergen Gross <jgross@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Julien Grall <julien@....org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [RFC PATCH 2/6] virtio: add option to restrict memory access under Xen

On April 14, 2022 12:19:29 PM PDT, Oleksandr Tyshchenko <olekstysh@...il.com> wrote:
>From: Juergen Gross <jgross@...e.com>
>
>In order to support virtio in Xen guests add a config option enabling
>the user to specify whether in all Xen guests virtio should be able to
>access memory via Xen grant mappings only on the host side.
>
>This applies to fully virtualized guests only, as for paravirtualized
>guests this is mandatory.
>
>This requires to switch arch_has_restricted_virtio_memory_access()
>from a pure stub to a real function on x86 systems (Arm systems are
>not covered by now).
>
>Add the needed functionality by providing a special set of DMA ops
>handling the needed grant operations for the I/O pages.
>
>Signed-off-by: Juergen Gross <jgross@...e.com>
>---
> arch/x86/mm/init.c        |  15 ++++
> arch/x86/mm/mem_encrypt.c |   5 --
> arch/x86/xen/Kconfig      |   9 +++
> drivers/xen/Kconfig       |  20 ++++++
> drivers/xen/Makefile      |   1 +
> drivers/xen/xen-virtio.c  | 177 ++++++++++++++++++++++++++++++++++++++++++++++
> include/xen/xen-ops.h     |   8 +++
> 7 files changed, 230 insertions(+), 5 deletions(-)
> create mode 100644 drivers/xen/xen-virtio.c
>
>diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>index d8cfce2..526a3b2 100644
>--- a/arch/x86/mm/init.c
>+++ b/arch/x86/mm/init.c
>@@ -8,6 +8,8 @@
> #include <linux/kmemleak.h>
> #include <linux/sched/task.h>
> 
>+#include <xen/xen.h>
>+
> #include <asm/set_memory.h>
> #include <asm/e820/api.h>
> #include <asm/init.h>
>@@ -1065,3 +1067,16 @@ unsigned long max_swapfile_size(void)
> 	return pages;
> }
> #endif
>+
>+#ifdef CONFIG_ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
>+int arch_has_restricted_virtio_memory_access(void)
>+{
>+	if (IS_ENABLED(CONFIG_XEN_PV_VIRTIO) && xen_pv_domain())
>+		return 1;
>+	if (IS_ENABLED(CONFIG_XEN_HVM_VIRTIO_GRANT) && xen_hvm_domain())
>+		return 1;
>+
>+	return cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
>+}
>+EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
>+#endif
>diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
>index 50d2099..dda020f 100644
>--- a/arch/x86/mm/mem_encrypt.c
>+++ b/arch/x86/mm/mem_encrypt.c
>@@ -77,8 +77,3 @@ void __init mem_encrypt_init(void)
> 	print_mem_encrypt_feature_info();
> }
> 
>-int arch_has_restricted_virtio_memory_access(void)
>-{
>-	return cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
>-}
>-EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
>diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
>index 85246dd..dffdffd 100644
>--- a/arch/x86/xen/Kconfig
>+++ b/arch/x86/xen/Kconfig
>@@ -92,3 +92,12 @@ config XEN_DOM0
> 	select X86_X2APIC if XEN_PVH && X86_64
> 	help
> 	  Support running as a Xen Dom0 guest.
>+
>+config XEN_PV_VIRTIO
>+	bool "Xen virtio support for PV guests"
>+	depends on XEN_VIRTIO && XEN_PV
>+	default y
>+	help
>+	  Support virtio for running as a paravirtualized guest. This will
>+	  need support on the backend side (qemu or kernel, depending on the
>+	  virtio device types used).
>diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>index 120d32f..fc61f7a 100644
>--- a/drivers/xen/Kconfig
>+++ b/drivers/xen/Kconfig
>@@ -335,4 +335,24 @@ config XEN_UNPOPULATED_ALLOC
> 	  having to balloon out RAM regions in order to obtain physical memory
> 	  space to create such mappings.
> 
>+config XEN_VIRTIO
>+	bool "Xen virtio support"
>+	default n
>+	depends on VIRTIO && DMA_OPS
>+	select ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
>+	help
>+	  Enable virtio support for running as Xen guest. Depending on the
>+	  guest type this will require special support on the backend side
>+	  (qemu or kernel, depending on the virtio device types used).
>+
>+config XEN_HVM_VIRTIO_GRANT
>+	bool "Require virtio for fully virtualized guests to use grant mappings"
>+	depends on XEN_VIRTIO && X86_64
>+	default y
>+	help
>+	  Require virtio for fully virtualized guests to use grant mappings.
>+	  This will avoid the need to give the backend the right to map all
>+	  of the guest memory. This will need support on the backend side
>+	  (qemu or kernel, depending on the virtio device types used).
>+
> endmenu
>diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
>index 5aae66e..767009c 100644
>--- a/drivers/xen/Makefile
>+++ b/drivers/xen/Makefile
>@@ -39,3 +39,4 @@ xen-gntalloc-y				:= gntalloc.o
> xen-privcmd-y				:= privcmd.o privcmd-buf.o
> obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)	+= xen-front-pgdir-shbuf.o
> obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)	+= unpopulated-alloc.o
>+obj-$(CONFIG_XEN_VIRTIO)		+= xen-virtio.o
>diff --git a/drivers/xen/xen-virtio.c b/drivers/xen/xen-virtio.c
>new file mode 100644
>index 00000000..cfd5eda
>--- /dev/null
>+++ b/drivers/xen/xen-virtio.c
>@@ -0,0 +1,177 @@
>+// SPDX-License-Identifier: GPL-2.0-only
>+/******************************************************************************
>+ * Xen virtio driver - enables using virtio devices in Xen guests.
>+ *
>+ * Copyright (c) 2021, Juergen Gross <jgross@...e.com>
>+ */
>+
>+#include <linux/module.h>
>+#include <linux/dma-map-ops.h>
>+#include <linux/pci.h>
>+#include <linux/pfn.h>
>+#include <linux/virtio_config.h>
>+#include <xen/xen.h>
>+#include <xen/grant_table.h>
>+
>+#define XEN_GRANT_ADDR_OFF	0x8000000000000000ULL
>+
>+static inline dma_addr_t grant_to_dma(grant_ref_t grant)
>+{
>+	return XEN_GRANT_ADDR_OFF | ((dma_addr_t)grant << PAGE_SHIFT);
>+}
>+
>+static inline grant_ref_t dma_to_grant(dma_addr_t dma)
>+{
>+	return (grant_ref_t)((dma & ~XEN_GRANT_ADDR_OFF) >> PAGE_SHIFT);
>+}
>+
>+/*
>+ * DMA ops for Xen virtio frontends.
>+ *
>+ * Used to act as a kind of software IOMMU for Xen guests by using grants as
>+ * DMA addresses.
>+ * Such a DMA address is formed by using the grant reference as a frame
>+ * number and setting the highest address bit (this bit is for the backend
>+ * to be able to distinguish it from e.g. a mmio address).
>+ *
>+ * Note that for now we hard wire dom0 to be the backend domain. In order to
>+ * support any domain as backend we'd need to add a way to communicate the
>+ * domid of this backend, e.g. via Xenstore or via the PCI-device's config
>+ * space.
>+ */
>+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;
>+	unsigned long pfn;
>+	grant_ref_t grant;
>+	void *ret;
>+
>+	ret = (void *)__get_free_pages(gfp, get_order(size));
>+	if (!ret)
>+		return NULL;
>+
>+	pfn = virt_to_pfn(ret);
>+
>+	if (gnttab_alloc_grant_reference_seq(n_pages, &grant)) {
>+		free_pages((unsigned long)ret, get_order(size));
>+		return NULL;
>+	}
>+
>+	for (i = 0; i < n_pages; i++) {
>+		gnttab_grant_foreign_access_ref(grant + i, 0,
>+						pfn_to_gfn(pfn + i), 0);
>+	}
>+
>+	*dma_handle = grant_to_dma(grant);
>+
>+	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;
>+	grant_ref_t grant;
>+
>+	grant = dma_to_grant(dma_handle);
>+
>+	for (i = 0; i < n_pages; i++)
>+		gnttab_end_foreign_access_ref(grant + i);
>+
>+	gnttab_free_grant_reference_seq(grant, n_pages);
>+
>+	free_pages((unsigned long)vaddr, get_order(size));
>+}
>+
>+static struct page *xen_virtio_dma_alloc_pages(struct device *dev, size_t size,
>+					       dma_addr_t *dma_handle,
>+					       enum dma_data_direction dir,
>+					       gfp_t gfp)
>+{
>+	WARN_ONCE(1, "xen_virtio_dma_alloc_pages size %ld\n", size);
>+	return NULL;
>+}
>+
>+static void xen_virtio_dma_free_pages(struct device *dev, size_t size,
>+				      struct page *vaddr, dma_addr_t dma_handle,
>+				      enum dma_data_direction dir)
>+{
>+	WARN_ONCE(1, "xen_virtio_dma_free_pages size %ld\n", size);
>+}
>+
>+static dma_addr_t xen_virtio_dma_map_page(struct device *dev, struct page *page,
>+					  unsigned long offset, size_t size,
>+					  enum dma_data_direction dir,
>+					  unsigned long attrs)
>+{
>+	grant_ref_t grant;
>+
>+	if (gnttab_alloc_grant_references(1, &grant))
>+		return 0;
>+
>+	gnttab_grant_foreign_access_ref(grant, 0, xen_page_to_gfn(page),
>+					dir == DMA_TO_DEVICE);
>+
>+	return grant_to_dma(grant) + offset;
>+}
>+
>+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)
>+{
>+	grant_ref_t grant;
>+
>+	grant = dma_to_grant(dma_handle);
>+
>+	gnttab_end_foreign_access_ref(grant);
>+
>+	gnttab_free_grant_reference(grant);
>+}
>+
>+static int xen_virtio_dma_map_sg(struct device *dev, struct scatterlist *sg,
>+				 int nents, enum dma_data_direction dir,
>+				 unsigned long attrs)
>+{
>+	WARN_ONCE(1, "xen_virtio_dma_map_sg nents %d\n", nents);
>+	return -EINVAL;
>+}
>+
>+static void xen_virtio_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
>+				    int nents, enum dma_data_direction dir,
>+				    unsigned long attrs)
>+{
>+	WARN_ONCE(1, "xen_virtio_dma_unmap_sg nents %d\n", nents);
>+}
>+
>+static int xen_virtio_dma_dma_supported(struct device *dev, u64 mask)
>+{
>+	return 1;
>+}
>+
>+static const struct dma_map_ops xen_virtio_dma_ops = {
>+	.alloc = xen_virtio_dma_alloc,
>+	.free = xen_virtio_dma_free,
>+	.alloc_pages = xen_virtio_dma_alloc_pages,
>+	.free_pages = xen_virtio_dma_free_pages,
>+	.mmap = dma_common_mmap,
>+	.get_sgtable = dma_common_get_sgtable,
>+	.map_page = xen_virtio_dma_map_page,
>+	.unmap_page = xen_virtio_dma_unmap_page,
>+	.map_sg = xen_virtio_dma_map_sg,
>+	.unmap_sg = xen_virtio_dma_unmap_sg,
>+	.dma_supported = xen_virtio_dma_dma_supported,
>+};
>+
>+void xen_virtio_setup_dma_ops(struct device *dev)
>+{
>+	dev->dma_ops = &xen_virtio_dma_ops;
>+}
>+EXPORT_SYMBOL_GPL(xen_virtio_setup_dma_ops);
>+
>+MODULE_DESCRIPTION("Xen virtio support driver");
>+MODULE_AUTHOR("Juergen Gross <jgross@...e.com>");
>+MODULE_LICENSE("GPL");
>diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
>index a3584a3..ae3c1bc 100644
>--- a/include/xen/xen-ops.h
>+++ b/include/xen/xen-ops.h
>@@ -221,4 +221,12 @@ static inline void xen_preemptible_hcall_end(void) { }
> 
> #endif /* CONFIG_XEN_PV && !CONFIG_PREEMPTION */
> 
>+#ifdef CONFIG_XEN_VIRTIO
>+void xen_virtio_setup_dma_ops(struct device *dev);
>+#else
>+static inline void xen_virtio_setup_dma_ops(struct device *dev)
>+{
>+}
>+#endif /* CONFIG_XEN_VIRTIO */
>+
> #endif /* INCLUDE_XEN_OPS_H */

Can you please encapsulate the Xen part of the test in some Xen-specific file?

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