[Xen-devel] [RFC PATCH v3 07/10] arm/mem_access: Add software guest-page-table walk

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

The function p2m_mem_access_check_and_get_page in mem_access.c
translates a gva to an ipa by means of the hardware functionality of the
ARM architecture. This is implemented in the function gva_to_ipa. If
mem_access is active, hardware-based gva to ipa translation might fail,
as gva_to_ipa uses the guest's translation tables, access to which might
be restricted by the active VTTBR. To address this issue, in this commit
we add a software-based guest-page-table walk, which will be used by the
function p2m_mem_access_check_and_get_page perform the gva to ipa
translation in software in one of the following commits.

Note: The introduced function guest_walk_tables assumes that the domain,
the gva of which is to be translated, is running on the currently active
vCPU. To walk the guest's page tables on a different vCPU, the following
registers would need to be loaded: TCR_EL1, TTBR0_EL1, TTBR1_EL1, and
SCTLR_EL1.

Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
---
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
---
v2: Rename p2m_gva_to_ipa to p2m_walk_gpt and move it to p2m.c.

    Move the functionality responsible for walking long-descriptor based
    translation tables out of the function p2m_walk_gpt. Also move out
    the long-descriptor based translation out of this commit.

    Change function parameters in order to return access access rights
    to a requested gva.

    Cosmetic fixes.

v3: Rename the introduced functions to guest_walk_(tables|sd|ld) and
    move the implementation to guest_copy.(c|h).

    Set permissions in guest_walk_tables also if the MMU is disabled.

    Change the function parameter of type "struct p2m_domain *" to
    "struct vcpu *" in the functions guest_walk_(tables|sd|ld).
---
 xen/arch/arm/Makefile            |  1 +
 xen/arch/arm/guest_walk.c        | 92 ++++++++++++++++++++++++++++++++++++++++
 xen/include/asm-arm/guest_walk.h | 19 +++++++++
 3 files changed, 112 insertions(+)
 create mode 100644 xen/arch/arm/guest_walk.c
 create mode 100644 xen/include/asm-arm/guest_walk.h

diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
index 49e1fb2f84..282d2c2949 100644
--- a/xen/arch/arm/Makefile
+++ b/xen/arch/arm/Makefile
@@ -21,6 +21,7 @@ obj-$(CONFIG_HAS_GICV3) += gic-v3.o
 obj-$(CONFIG_HAS_ITS) += gic-v3-its.o
 obj-$(CONFIG_HAS_ITS) += gic-v3-lpi.o
 obj-y += guestcopy.o
+obj-y += guest_walk.o
 obj-y += hvm.o
 obj-y += io.o
 obj-y += irq.o
diff --git a/xen/arch/arm/guest_walk.c b/xen/arch/arm/guest_walk.c
new file mode 100644
index 0000000000..f2f3533665
--- /dev/null
+++ b/xen/arch/arm/guest_walk.c
@@ -0,0 +1,92 @@
+/*
+ * Guest page table walk
+ * Copyright (c) 2017 Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <xen/sched.h>
+
+/*
+ * The function guest_walk_sd translates a given GVA into an IPA using the
+ * short-descriptor translation table format in software. This function assumes
+ * that the domain is running on the currently active vCPU. To walk the guest's
+ * page table on a different vCPU, the following registers would need to be
+ * loaded: TCR_EL1, TTBR0_EL1, TTBR1_EL1, and SCTLR_EL1.
+ */
+static int guest_walk_sd(struct domain *d,
+                         vaddr_t gva, paddr_t *ipa,
+                         unsigned int *perms)
+{
+    /* Not implemented yet. */
+    return -EFAULT;
+}
+
+/*
+ * The function guest_walk_ld translates a given GVA into an IPA using the
+ * long-descriptor translation table format in software. This function assumes
+ * that the domain is running on the currently active vCPU. To walk the guest's
+ * page table on a different vCPU, the following registers would need to be
+ * loaded: TCR_EL1, TTBR0_EL1, TTBR1_EL1, and SCTLR_EL1.
+ */
+static int guest_walk_ld(struct domain *d,
+                         vaddr_t gva, paddr_t *ipa,
+                         unsigned int *perms)
+{
+    /* Not implemented yet. */
+    return -EFAULT;
+}
+
+int guest_walk_tables(const struct vcpu *v, vaddr_t gva,
+                      paddr_t *ipa, unsigned int *perms)
+{
+    uint32_t sctlr = READ_SYSREG(SCTLR_EL1);
+    register_t tcr = READ_SYSREG(TCR_EL1);
+    struct domain *d = v->domain;
+    unsigned int _perms = GV2M_READ;
+
+    /* We assume that the domain is running on the currently active domain. */
+    if ( v != current )
+        return -EFAULT;
+
+    /* Allow perms to be NULL. */
+    perms = perms ?: &_perms;
+
+    /* If the MMU is disabled, there is no need to translate the gva. */
+    if ( !(sctlr & SCTLR_M) )
+    {
+        *ipa = gva;
+
+        /* Memory can be accessed without any restrictions. */
+        *perms = GV2M_READ|GV2M_WRITE|GV2M_EXEC;
+
+        return 0;
+    }
+
+    if ( is_32bit_domain(d) )
+    {
+        if ( !(tcr & TTBCR_EAE) )
+            return guest_walk_sd(d, gva, ipa, perms);
+    }
+
+    return guest_walk_ld(d, gva, ipa, perms);
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/include/asm-arm/guest_walk.h b/xen/include/asm-arm/guest_walk.h
new file mode 100644
index 0000000000..4ed8476e08
--- /dev/null
+++ b/xen/include/asm-arm/guest_walk.h
@@ -0,0 +1,19 @@
+#ifndef _XEN_GUEST_WALK_H
+#define _XEN_GUEST_WALK_H
+
+/* Walk the guest's page tables in software. */
+int guest_walk_tables(const struct vcpu *v,
+                      vaddr_t gva,
+                      paddr_t *ipa,
+                      unsigned int *perms);
+
+#endif /* _XEN_GUEST_WALK_H */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
-- 
2.12.2


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