Re: [PATCH v3 3/9] arm64: KVM: add trap handlers for AArch64 debug registers

Christoffer Dall
2014-07-09T14:52:17Z

On Wed, Jul 09, 2014 at 12:09:29PM +0100, Marc Zyngier wrote:
> On Wed, Jul 09 2014 at 10:38:13 am BST, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> > On Fri, Jun 20, 2014 at 02:00:01PM +0100, Marc Zyngier wrote:
> >> Add handlers for all the AArch64 debug registers that are accessible
> >> from EL0 or EL1. The trapping code keeps track of the state of the
> >> debug registers, allowing for the switch code to implement a lazy
> >> switching strategy.
> >>
> >> Reviewed-by: Anup Patel <anup.patel@linaro.org>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >> arch/arm64/include/asm/kvm_asm.h | 28 ++++++--
> >> arch/arm64/include/asm/kvm_host.h | 3 +
> >> arch/arm64/kvm/sys_regs.c | 137 +++++++++++++++++++++++++++++++++++++-
> >> 3 files changed, 159 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> >> index 9fcd54b..e6b159a 100644
> >> --- a/arch/arm64/include/asm/kvm_asm.h
> >> +++ b/arch/arm64/include/asm/kvm_asm.h
> >> @@ -43,14 +43,25 @@
> >> #define AMAIR_EL1 19 /* Aux Memory Attribute Indirection Register */
> >> #define CNTKCTL_EL1 20 /* Timer Control Register (EL1) */
> >> #define PAR_EL1 21 /* Physical Address Register */
> >> +#define MDSCR_EL1 22 /* Monitor Debug System Control Register */
> >> +#define DBGBCR0_EL1 23 /* Debug Breakpoint Control Registers (0-15) */
> >> +#define DBGBCR15_EL1 38
> >> +#define DBGBVR0_EL1 39 /* Debug Breakpoint Value Registers (0-15) */
> >> +#define DBGBVR15_EL1 54
> >> +#define DBGWCR0_EL1 55 /* Debug Watchpoint Control Registers (0-15) */
> >> +#define DBGWCR15_EL1 70
> >> +#define DBGWVR0_EL1 71 /* Debug Watchpoint Value Registers (0-15) */
> >> +#define DBGWVR15_EL1 86
> >> +#define MDCCINT_EL1 87 /* Monitor Debug Comms Channel Interrupt Enable Reg */
> >> +
> >> /* 32bit specific registers. Keep them at the end of the range */
> >> -#define DACR32_EL2 22 /* Domain Access Control Register */
> >> -#define IFSR32_EL2 23 /* Instruction Fault Status Register */
> >> -#define FPEXC32_EL2 24 /* Floating-Point Exception Control Register */
> >> -#define DBGVCR32_EL2 25 /* Debug Vector Catch Register */
> >> -#define TEECR32_EL1 26 /* ThumbEE Configuration Register */
> >> -#define TEEHBR32_EL1 27 /* ThumbEE Handler Base Register */
> >> -#define NR_SYS_REGS 28
> >> +#define DACR32_EL2 88 /* Domain Access Control Register */
> >> +#define IFSR32_EL2 89 /* Instruction Fault Status Register */
> >> +#define FPEXC32_EL2 90 /* Floating-Point Exception Control Register */
> >> +#define DBGVCR32_EL2 91 /* Debug Vector Catch Register */
> >> +#define TEECR32_EL1 92 /* ThumbEE Configuration Register */
> >> +#define TEEHBR32_EL1 93 /* ThumbEE Handler Base Register */
> >> +#define NR_SYS_REGS 94
> >>
> >> /* 32bit mapping */
> >> #define c0_MPIDR (MPIDR_EL1 * 2) /* MultiProcessor ID Register */
> >> @@ -87,6 +98,9 @@
> >> #define ARM_EXCEPTION_IRQ 0
> >> #define ARM_EXCEPTION_TRAP 1
> >>
> >> +#define KVM_ARM64_DEBUG_DIRTY_SHIFT 0
> >> +#define KVM_ARM64_DEBUG_DIRTY (1 << KVM_ARM64_DEBUG_DIRTY_SHIFT)
> >> +
> >> #ifndef __ASSEMBLY__
> >> struct kvm;
> >> struct kvm_vcpu;
> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> index 92242ce..79573c86 100644
> >> --- a/arch/arm64/include/asm/kvm_host.h
> >> +++ b/arch/arm64/include/asm/kvm_host.h
> >> @@ -101,6 +101,9 @@ struct kvm_vcpu_arch {
> >> /* Exception Information */
> >> struct kvm_vcpu_fault_info fault;
> >>
> >> + /* Debug state */
> >> + u64 debug_flags;
> >> +
> >> /* Pointer to host CPU context */
> >> kvm_cpu_context_t *host_cpu_context;
> >>
> >> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> >> index 4abd84e..808e3b2 100644
> >> --- a/arch/arm64/kvm/sys_regs.c
> >> +++ b/arch/arm64/kvm/sys_regs.c
> >> @@ -30,6 +30,7 @@
> >> #include <asm/kvm_mmu.h>
> >> #include <asm/cacheflush.h>
> >> #include <asm/cputype.h>
> >> +#include <asm/debug-monitors.h>
> >> #include <trace/events/kvm.h>
> >>
> >> #include "sys_regs.h"
> >> @@ -173,6 +174,60 @@ static bool trap_raz_wi(struct kvm_vcpu *vcpu,
> >> return read_zero(vcpu, p);
> >> }
> >>
> >> +static bool trap_oslsr_el1(struct kvm_vcpu *vcpu,
> >> + const struct sys_reg_params *p,
> >> + const struct sys_reg_desc *r)
> >> +{
> >> + if (p->is_write) {
> >> + return ignore_write(vcpu, p);
> >> + } else {
> >> + *vcpu_reg(vcpu, p->Rt) = (1 << 3);
> >> + return true;
> >> + }
> >> +}
> >> +
> >> +static bool trap_dbgauthstatus_el1(struct kvm_vcpu *vcpu,
> >> + const struct sys_reg_params *p,
> >> + const struct sys_reg_desc *r)
> >> +{
> >> + if (p->is_write) {
> >> + return ignore_write(vcpu, p);
> >> + } else {
> >> + u32 val;
> >> + asm volatile("mrs %0, dbgauthstatus_el1" : "=r" (val));
> >> + *vcpu_reg(vcpu, p->Rt) = val;
> >> + return true;
> >> + }
> >> +}
> >> +
> >> +/*
> >> + * We want to avoid world-switching all the DBG registers all the
> >> + * time. For this, we use a DIRTY but, indicating the guest has
> >
> > a DIRTY but? (at least there's only one t in there).
>
> The whole debug architecture makes me feel very dirty.
>
> >> + * modified the debug registers, and only restore the registers once,
> >> + * disabling traps.
> >
> > I don't think I understand the "only restore the registers once" bit
> > here. I know I'm being incredibly stupid, but I forgot since the last
> > review round how this actually works; when we return from the guest and
> > the guest has somehow enabled certain DBG functionality, then we set the dirty
> > flag, which means we should stop trapping and context switch all the
> > registers on world-switches, but if we see when returning from the guest
> > that the guest doesn't appear to be using the registers we enable
> > trapping and stop world-switching, right?
>
> Almost. We always decide on the trapping when entering the guest:
> - If the dirty bit is set (because we're coming back from trapping),
> disable the traps, restore the registers
> - If debug is actively in use (DBG_MDSCR_KDE or DBG_MDSCR_MDE set),
> disable the traps, restore the registers

this also sets the dirty bit then?

> - Otherwise, enable the traps
>
> When exiting the guest: If the dirty bit is set, save the registers and
> clear the dirty bit.

because the host should always be able to freely use the debug registers
so we always have to restore the host registers on coming out of the VM,
right?

>
> > Do we clearly define which state triggers the world-switching and why
> > that's a good rationale? (sorry, the debug architecture is not my
> > favorite part of the ARM ARM).
>
> I thing the above comment describes the state precisely. My rational is:
> - If we've touched any debug register, it is likely that we're going to
> touch more of them. It then makes sense to disable the traps and start
> doing the save/restore dance
> - If debug is active (DBG_MDSCR_KDE or DBG_MDSCR_MDE set), it is then
> mandatory to save/restore the registers, as the guest depends on them.
>
> Does this make the process clearer? If so, I can add it to the comment.
>

yes, the above comments help a lot. thanks!!

[if I don't see your response because you already left for vacation, I'm
going to incorporate your comments in the patches to apply to
kvmarm/next].

> >> + *
> >> + * The best thing to do would be to trap MDSCR_EL1 independently, test
> >> + * if DBG_MDSCR_KDE or DBG_MDSCR_MDE is getting set, and only set the
> >> + * DIRTY bit in that case.
> >> + *
> >> + * Unfortunately, "old" Linux kernels tend to hit MDSCR_EL1 like a
> >> + * woodpecker on a tree, and it is better to disable trapping as soon
> >> + * as possible in this case. Some day, make this a tuneable...
> >> + */
> >> +static bool trap_debug_regs(struct kvm_vcpu *vcpu,
> >> + const struct sys_reg_params *p,
> >> + const struct sys_reg_desc *r)
> >> +{
> >> + if (p->is_write) {
> >> + vcpu_sys_reg(vcpu, r->reg) = *vcpu_reg(vcpu, p->Rt);
> >> + vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> >> + } else {
> >> + *vcpu_reg(vcpu, p->Rt) = vcpu_sys_reg(vcpu, r->reg);
> >> + }
> >> +
> >> + return true;
> >> +}
> >> +
> >> static void reset_amair_el1(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> >> {
> >> u64 amair;
> >> @@ -189,6 +244,21 @@ static void reset_mpidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> >> vcpu_sys_reg(vcpu, MPIDR_EL1) = (1UL << 31) | (vcpu->vcpu_id & 0xff);
> >> }
> >>
> >> +/* Silly macro to expand the DBG{BCR,BVR,WVR,WCR}n_EL1 registers in one go */
> >> +#define DBG_BCR_BVR_WCR_WVR_EL1(n) \
> >> + /* DBGBVRn_EL1 */ \
> >> + { Op0(0b10), Op1(0b000), CRn(0b0000), CRm((n)), Op2(0b100), \
> >> + trap_debug_regs, reset_val, (DBGBVR0_EL1 + (n)), 0 }, \
> >> + /* DBGBCRn_EL1 */ \
> >> + { Op0(0b10), Op1(0b000), CRn(0b0000), CRm((n)), Op2(0b101), \
> >> + trap_debug_regs, reset_val, (DBGBCR0_EL1 + (n)), 0 }, \
> >> + /* DBGWVRn_EL1 */ \
> >> + { Op0(0b10), Op1(0b000), CRn(0b0000), CRm((n)), Op2(0b110), \
> >> + trap_debug_regs, reset_val, (DBGWVR0_EL1 + (n)), 0 }, \
> >> + /* DBGWCRn_EL1 */ \
> >> + { Op0(0b10), Op1(0b000), CRn(0b0000), CRm((n)), Op2(0b111), \
> >> + trap_debug_regs, reset_val, (DBGWCR0_EL1 + (n)), 0 }
> >> +
> >> /*
> >> * Architected system registers.
> >> * Important: Must be sorted ascending by Op0, Op1, CRn, CRm, Op2
> >> @@ -201,8 +271,12 @@ static void reset_mpidr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> >> * must always support PMCCNTR (the cycle counter): we just RAZ/WI for
> >> * all PM registers, which doesn't crash the guest kernel at least.
> >> *
> >> - * Same goes for the whole debug infrastructure, which probably breaks
> >> - * some guest functionnality. This should be fixed.
> >> + * Debug handling: We do trap most, if not all debug related system
> >> + * registers. The implementation is good enough to ensure that a guest
> >> + * can use these with minimal performance degradation. The drawback is
> >> + * that we don't implement any of the external debug, none of the
> >> + * OSlock protocol. This should be revisited if we ever encounter a
> >> + * more demanding guest...
> >> */
> >> static const struct sys_reg_desc sys_reg_descs[] = {
> >> /* DC ISW */
> >> @@ -215,12 +289,71 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >> { Op0(0b01), Op1(0b000), CRn(0b0111), CRm(0b1110), Op2(0b010),
> >> access_dcsw },
> >>
> >> + DBG_BCR_BVR_WCR_WVR_EL1(0),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(1),
> >> + /* MDCCINT_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0000), CRm(0b0010), Op2(0b000),
> >> + trap_debug_regs, reset_val, MDCCINT_EL1, 0 },
> >> + /* MDSCR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0000), CRm(0b0010), Op2(0b010),
> >> + trap_debug_regs, reset_val, MDSCR_EL1, 0 },
> >> + DBG_BCR_BVR_WCR_WVR_EL1(2),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(3),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(4),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(5),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(6),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(7),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(8),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(9),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(10),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(11),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(12),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(13),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(14),
> >> + DBG_BCR_BVR_WCR_WVR_EL1(15),
> >> +
> >> + /* MDRAR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0001), CRm(0b0000), Op2(0b000),
> >> + trap_raz_wi },
> >> + /* OSLAR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0001), CRm(0b0000), Op2(0b100),
> >> + trap_raz_wi },
> >> + /* OSLSR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0001), CRm(0b0001), Op2(0b100),
> >> + trap_oslsr_el1 },
> >> + /* OSDLR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0001), CRm(0b0011), Op2(0b100),
> >> + trap_raz_wi },
> >> + /* DBGPRCR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0001), CRm(0b0100), Op2(0b100),
> >> + trap_raz_wi },
> >> + /* DBGCLAIMSET_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0111), CRm(0b1000), Op2(0b110),
> >> + trap_raz_wi },
> >> + /* DBGCLAIMCLR_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0111), CRm(0b1001), Op2(0b110),
> >> + trap_raz_wi },
> >> + /* DBGAUTHSTATUS_EL1 */
> >> + { Op0(0b10), Op1(0b000), CRn(0b0111), CRm(0b1110), Op2(0b110),
> >> + trap_dbgauthstatus_el1 },
> >> +
> >> /* TEECR32_EL1 */
> >> { Op0(0b10), Op1(0b010), CRn(0b0000), CRm(0b0000), Op2(0b000),
> >> NULL, reset_val, TEECR32_EL1, 0 },
> >> /* TEEHBR32_EL1 */
> >> { Op0(0b10), Op1(0b010), CRn(0b0001), CRm(0b0000), Op2(0b000),
> >> NULL, reset_val, TEEHBR32_EL1, 0 },
> >> +
> >> + /* MDCCSR_EL1 */
> >> + { Op0(0b10), Op1(0b011), CRn(0b0000), CRm(0b0001), Op2(0b000),
> >> + trap_raz_wi },
> >> + /* DBGDTR_EL0 */
> >> + { Op0(0b10), Op1(0b011), CRn(0b0000), CRm(0b0100), Op2(0b000),
> >> + trap_raz_wi },
> >> + /* DBGDTR[TR]X_EL0 */
> >> + { Op0(0b10), Op1(0b011), CRn(0b0000), CRm(0b0101), Op2(0b000),
> >> + trap_raz_wi },
> >> +
> >> /* DBGVCR32_EL2 */
> >> { Op0(0b10), Op1(0b100), CRn(0b0000), CRm(0b0111), Op2(0b000),
> >> NULL, reset_val, DBGVCR32_EL2, 0 },
> >> --
> >> 1.8.3.4
> >>
> >
> > Besides the commenting stuff above:
> >
> > Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
>
> Thanks,
>
> M.
> --
> Without deviation from the norm, progress is not possible.