[Xen-devel] [PATCH v1 10/13] xen/arm64: Implement a fast path for handling SMCCC_ARCH_WORKAROUND_2

[Julien Grall <julien.grall@xxxxxxx>]

The function ARM_SMCCC_ARCH_WORKAROUND_2 will be called by the guest for
enabling/disabling the ssbd mitigation. So we want the handling to
be as fast as possible.

The new sequence will forward guest's ARCH_WORKAROUND_2 call to EL3 and
also track the state of the workaround per-vCPU.

Note that since we need to execute branches, this always executes after
the spectre-v2 mitigation.

This code is based on KVM counterpart "arm64: KVM: Handle guest's
ARCH_WORKAROUND_2 requests" written by Marc Zyngier.

This is part of XSA-263.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---
    Changes in v2:
        - Combine the 2 consecutive eor instructions in a single one.
---
 xen/arch/arm/arm64/asm-offsets.c |  2 ++
 xen/arch/arm/arm64/entry.S       | 42 +++++++++++++++++++++++++++++++++++++++-
 xen/arch/arm/cpuerrata.c         | 18 +++++++++++++++++
 3 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/arm64/asm-offsets.c b/xen/arch/arm/arm64/asm-offsets.c
index ce24e44473..f5c696d092 100644
--- a/xen/arch/arm/arm64/asm-offsets.c
+++ b/xen/arch/arm/arm64/asm-offsets.c
@@ -22,6 +22,7 @@
 void __dummy__(void)
 {
    OFFSET(UREGS_X0, struct cpu_user_regs, x0);
+   OFFSET(UREGS_X1, struct cpu_user_regs, x1);
    OFFSET(UREGS_LR, struct cpu_user_regs, lr);
 
    OFFSET(UREGS_SP, struct cpu_user_regs, sp);
@@ -45,6 +46,7 @@ void __dummy__(void)
    BLANK();
 
    DEFINE(CPUINFO_sizeof, sizeof(struct cpu_info));
+   OFFSET(CPUINFO_flags, struct cpu_info, flags);
 
    OFFSET(VCPU_arch_saved_context, struct vcpu, arch.saved_context);
 
diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
index e2344e565f..97b05f53ea 100644
--- a/xen/arch/arm/arm64/entry.S
+++ b/xen/arch/arm/arm64/entry.S
@@ -1,4 +1,6 @@
 #include <asm/asm_defns.h>
+#include <asm/current.h>
+#include <asm/macros.h>
 #include <asm/regs.h>
 #include <asm/alternative.h>
 #include <asm/smccc.h>
@@ -241,7 +243,7 @@ guest_sync:
          * be encoded as an immediate for cmp.
          */
         eor     w0, w0, #ARM_SMCCC_ARCH_WORKAROUND_1_FID
-        cbnz    w0, guest_sync_slowpath
+        cbnz    w0, check_wa2
 
         /*
          * Clobber both x0 and x1 to prevent leakage. Note that thanks
@@ -250,6 +252,44 @@ guest_sync:
         mov     x1, xzr
         eret
 
+check_wa2:
+        /* ARM_SMCCC_ARCH_WORKAROUND_2 handling */
+        eor     w0, w0, #(ARM_SMCCC_ARCH_WORKAROUND_1_FID ^ 
ARM_SMCCC_ARCH_WORKAROUND_2_FID)
+        cbnz    w0, guest_sync_slowpath
+#ifdef CONFIG_ARM_SSBD
+alternative_cb arm_enable_wa2_handling
+        b       wa2_end
+alternative_cb_end
+        /* Sanitize the argument */
+        mov     x0, #-(UREGS_kernel_sizeof - UREGS_X1)  /* x0 := offset of 
guest's x1 on the stack */
+        ldr     x1, [sp, x0]                            /* Load guest's x1 */
+        cmp     w1, wzr
+        cset    x1, ne
+
+        /*
+         * Update the guest flag. At this stage sp point after the field
+         * guest_cpu_user_regs in cpu_info.
+         */
+        adr_cpu_info x2
+        ldr     x0, [x2, #CPUINFO_flags]
+        bfi     x0, x1, #CPUINFO_WORKAROUND_2_FLAG_SHIFT, #1
+        str     x0, [x2, #CPUINFO_flags]
+
+        /* Check that we actually need to perform the call */
+        ldr_this_cpu x0, ssbd_callback_required, x2
+        cbz     x0, wa2_end
+
+        mov     w0, #ARM_SMCCC_ARCH_WORKAROUND_2_FID
+        smc     #0
+
+wa2_end:
+        /* Don't leak data from the SMC call */
+        mov     x1, xzr
+        mov     x2, xzr
+        mov     x3, xzr
+#endif /* !CONFIG_ARM_SSBD */
+        mov     x0, xzr
+        eret
 guest_sync_slowpath:
         /*
          * x0/x1 may have been scratch by the fast path above, so avoid
diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
index 4292008692..7455f09f26 100644
--- a/xen/arch/arm/cpuerrata.c
+++ b/xen/arch/arm/cpuerrata.c
@@ -7,6 +7,7 @@
 #include <xen/warning.h>
 #include <asm/cpufeature.h>
 #include <asm/cpuerrata.h>
+#include <asm/insn.h>
 #include <asm/psci.h>
 
 /* Override macros from asm/page.h to make them work with mfn_t */
@@ -272,6 +273,23 @@ static int __init parse_spec_ctrl(const char *s)
 }
 custom_param("spec-ctrl", parse_spec_ctrl);
 
+/* Arm64 only for now as for Arm32 the workaround is currently handled in C. */
+#ifdef CONFIG_ARM_64
+void __init arm_enable_wa2_handling(const struct alt_instr *alt,
+                                    const uint32_t *origptr,
+                                    uint32_t *updptr, int nr_inst)
+{
+    BUG_ON(nr_inst != 1);
+
+    /*
+     * Only allow mitigation on guest ARCH_WORKAROUND_2 if the SSBD
+     * state allow it to be flipped.
+     */
+    if ( get_ssbd_state() == ARM_SSBD_RUNTIME )
+        *updptr = aarch64_insn_gen_nop();
+}
+#endif
+
 /*
  * Assembly code may use the variable directly, so we need to make sure
  * it fits in a register.
-- 
2.11.0


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