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Re: [PATCH v4 1/2] arm64: kvm: Save/restore MTE registers

From: Marc Zyngier
Subject: Re: [PATCH v4 1/2] arm64: kvm: Save/restore MTE registers
Date: Tue, 17 Nov 2020 19:20:34 +0000
User-agent: Roundcube Webmail/1.4.9 
Hi Steven,

These patches unfortunately don't apply to -rc4 anymore, as we repainted
quite a bit while working on fixes. I'd be grateful if you could rebase them. 

A few other things though:

On 2020-10-26 15:57, Steven Price wrote:

Define the new system registers that MTE introduces and context switch
them. The MTE feature is still hidden from the ID register as it isn't
supported in a VM yet.

Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
---
 arch/arm64/include/asm/kvm_host.h          |  4 ++++
 arch/arm64/include/asm/sysreg.h            |  3 ++-
 arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 14 ++++++++++++++
 arch/arm64/kvm/sys_regs.c                  | 14 ++++++++++----
 4 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h
b/arch/arm64/include/asm/kvm_host.h
index 0aecbab6a7fb..95ab7345dcc8 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -134,6 +134,8 @@ enum vcpu_sysreg {
        SCTLR_EL1,      /* System Control Register */
        ACTLR_EL1,      /* Auxiliary Control Register */
        CPACR_EL1,      /* Coprocessor Access Control */
+       RGSR_EL1,       /* Random Allocation Tag Seed Register */
+       GCR_EL1,        /* Tag Control Register */
        ZCR_EL1,        /* SVE Control */
        TTBR0_EL1,      /* Translation Table Base Register 0 */
        TTBR1_EL1,      /* Translation Table Base Register 1 */
@@ -150,6 +152,8 @@ enum vcpu_sysreg {
        TPIDR_EL1,      /* Thread ID, Privileged */
        AMAIR_EL1,      /* Aux Memory Attribute Indirection Register */
        CNTKCTL_EL1,    /* Timer Control Register (EL1) */
+       TFSRE0_EL1,     /* Tag Fault Status Register (EL0) */
+       TFSR_EL1,       /* Tag Fault Stauts Register (EL1) */
        PAR_EL1,        /* Physical Address Register */
        MDSCR_EL1,      /* Monitor Debug System Control Register */
        MDCCINT_EL1,    /* Monitor Debug Comms Channel Interrupt Enable Reg */
diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h 
index d52c1b3ce589..7727df0bc09d 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -565,7 +565,8 @@
 #define SCTLR_ELx_M    (BIT(0))

 #define SCTLR_ELx_FLAGS        (SCTLR_ELx_M  | SCTLR_ELx_A | SCTLR_ELx_C | \
-                        SCTLR_ELx_SA | SCTLR_ELx_I | SCTLR_ELx_IESB)
+                        SCTLR_ELx_SA | SCTLR_ELx_I | SCTLR_ELx_IESB | \
+                        SCTLR_ELx_ITFSB)

 /* SCTLR_EL2 specific flags. */
#define SCTLR_EL2_RES1 ((BIT(4)) | (BIT(5)) | (BIT(11)) | (BIT(16)) | \ 
diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
index 7a986030145f..a124ffa49ba3 100644
--- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
+++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
@@ -18,6 +18,11 @@
static inline void __sysreg_save_common_state(struct kvm_cpu_context *ctxt) 
 {
        ctxt_sys_reg(ctxt, MDSCR_EL1)   = read_sysreg(mdscr_el1);
+       if (system_supports_mte()) {
+               ctxt_sys_reg(ctxt, RGSR_EL1)    = read_sysreg_s(SYS_RGSR_EL1);
+               ctxt_sys_reg(ctxt, GCR_EL1)     = read_sysreg_s(SYS_GCR_EL1);
+               ctxt_sys_reg(ctxt, TFSRE0_EL1)  = read_sysreg_s(SYS_TFSRE0_EL1);


As far as I can tell, HCR_EL2.ATA is still clear when running a guest.
So why, do we save/restore this state yet?

Also, I wonder whether we should keep these in the C code. If one day
we enable MTE in the kernel, we will have to move them to the assembly
part, much like we do for PAuth. And I fear that "one day" is pretty
soon:

https://lore.kernel.org/linux-arm-kernel/cover.1605046192.git.andreyknvl@google.com/



+       }
 }
static inline void __sysreg_save_user_state(struct kvm_cpu_context *ctxt) 
@@ -45,6 +50,8 @@ static inline void __sysreg_save_el1_state(struct
kvm_cpu_context *ctxt)
        ctxt_sys_reg(ctxt, CNTKCTL_EL1) = read_sysreg_el1(SYS_CNTKCTL);
        ctxt_sys_reg(ctxt, PAR_EL1)     = read_sysreg(par_el1);
        ctxt_sys_reg(ctxt, TPIDR_EL1)   = read_sysreg(tpidr_el1);
+       if (system_supports_mte())
+               ctxt_sys_reg(ctxt, TFSR_EL1) = read_sysreg_el1(SYS_TFSR);

        ctxt_sys_reg(ctxt, SP_EL1)      = read_sysreg(sp_el1);
        ctxt_sys_reg(ctxt, ELR_EL1)     = read_sysreg_el1(SYS_ELR);
@@ -63,6 +70,11 @@ static inline void
__sysreg_save_el2_return_state(struct kvm_cpu_context *ctxt)
static inline void __sysreg_restore_common_state(struct kvm_cpu_context *ctxt) 
 {
        write_sysreg(ctxt_sys_reg(ctxt, MDSCR_EL1),  mdscr_el1);
+       if (system_supports_mte()) {
+               write_sysreg_s(ctxt_sys_reg(ctxt, RGSR_EL1), SYS_RGSR_EL1);
+               write_sysreg_s(ctxt_sys_reg(ctxt, GCR_EL1), SYS_GCR_EL1);
+               write_sysreg_s(ctxt_sys_reg(ctxt, TFSRE0_EL1), SYS_TFSRE0_EL1);
+       }
 }
static inline void __sysreg_restore_user_state(struct kvm_cpu_context *ctxt) 
@@ -106,6 +118,8 @@ static inline void
__sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
        write_sysreg_el1(ctxt_sys_reg(ctxt, CNTKCTL_EL1), SYS_CNTKCTL);
        write_sysreg(ctxt_sys_reg(ctxt, PAR_EL1),       par_el1);
        write_sysreg(ctxt_sys_reg(ctxt, TPIDR_EL1),     tpidr_el1);
+       if (system_supports_mte())
+               write_sysreg_el1(ctxt_sys_reg(ctxt, TFSR_EL1), SYS_TFSR);

        if (!has_vhe() &&
            cpus_have_final_cap(ARM64_WORKAROUND_SPECULATIVE_AT) &&
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index d9117bc56237..430e36e1a13d 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1391,6 +1391,12 @@ static bool access_mte_regs(struct kvm_vcpu
*vcpu, struct sys_reg_params *p,
        return false;
 }

+static unsigned int mte_visibility(const struct kvm_vcpu *vcpu,
+                                  const struct sys_reg_desc *rd)
+{
+       return REG_HIDDEN_USER | REG_HIDDEN_GUEST;


The handling of visibility has changed somehow since 01fe5ace92dd.


+}
+
 /* sys_reg_desc initialiser for known cpufeature ID registers */
 #define ID_SANITISED(name) {                   \
        SYS_DESC(SYS_##name),                   \
@@ -1557,8 +1563,8 @@ static const struct sys_reg_desc sys_reg_descs[] = { 
        { SYS_DESC(SYS_ACTLR_EL1), access_actlr, reset_actlr, ACTLR_EL1 },
        { SYS_DESC(SYS_CPACR_EL1), NULL, reset_val, CPACR_EL1, 0 },

-       { SYS_DESC(SYS_RGSR_EL1), access_mte_regs },
-       { SYS_DESC(SYS_GCR_EL1), access_mte_regs },
+       { SYS_DESC(SYS_RGSR_EL1), access_mte_regs, reset_unknown, RGSR_EL1,
.visibility = mte_visibility },
+       { SYS_DESC(SYS_GCR_EL1), access_mte_regs, reset_unknown, GCR_EL1,
.visibility = mte_visibility },

        { SYS_DESC(SYS_ZCR_EL1), NULL, reset_val, ZCR_EL1, 0, .visibility =
sve_visibility },
        { SYS_DESC(SYS_TTBR0_EL1), access_vm_reg, reset_unknown, TTBR0_EL1 },
@@ -1584,8 +1590,8 @@ static const struct sys_reg_desc sys_reg_descs[] = { 
        { SYS_DESC(SYS_ERXMISC0_EL1), trap_raz_wi },
        { SYS_DESC(SYS_ERXMISC1_EL1), trap_raz_wi },

-       { SYS_DESC(SYS_TFSR_EL1), access_mte_regs },
-       { SYS_DESC(SYS_TFSRE0_EL1), access_mte_regs },
+       { SYS_DESC(SYS_TFSR_EL1), access_mte_regs, reset_unknown, TFSR_EL1,
.visibility = mte_visibility },
+       { SYS_DESC(SYS_TFSRE0_EL1), access_mte_regs, reset_unknown,
TFSRE0_EL1, .visibility = mte_visibility },

        { SYS_DESC(SYS_FAR_EL1), access_vm_reg, reset_unknown, FAR_EL1 },
        { SYS_DESC(SYS_PAR_EL1), NULL, reset_unknown, PAR_EL1 },


Thanks,

        M.
--
Jazz is not dead. It just smells funny...
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