[PATCH v3 61/66] KVM: arm64: nv: Synchronize PSTATE early on exit

Marc Zyngier
2020-12-10T16:31:23Z

The NV code relies on predicates such as is_hyp_ctxt() being
reliable. In turn, is_hyp_ctxt() relies on things like PSTATE
and the virtual HCR_EL2 being accurate.

But with ARMv8.4-NV removing trapping for a large part of the
EL2 system registers (among which HCR_EL2), we can't use such
trapping to synchronize the rest of the state.

Let's look at the following sequence for a VHE guest:

(1) enter guest in host EL0
(2) guest traps to guest vEL2 (no hypervisor intervention)
(3) guest clears virtual HCR_EL2.TGE (no trap either)
(4) host interrupt fires, exit
(5) is_hyp_ctxt() now says "guest" (PSTATE.M==EL1 and TGE==0)

It is obvious that such behaviour would be rather unfortunate,
and lead to interesting, difficult to catch bugs specially if
preemption kicks in (yes, I wasted a whole week chasing this one).

In order to preserve the invariant that a guest entered in host
context must exit in the same context, we must make sure that
is_hyp_ctxt() works correctly. Since we can always observe the
guest value of HCR_EL2.{E2H,TGE} in the VNCR_EL2 page, we solely
need to synchronize PSTATE as early as possible.

This basically amounts to moving from_hw_pstate() as close
as possible to the guest exit point, and fixup_guest_exit()
seems as good a place as any.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/kvm/hyp/include/hyp/switch.h | 16 ++++--
arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 26 ++--------
arch/arm64/kvm/hyp/nvhe/switch.c | 8 ++-
arch/arm64/kvm/hyp/vhe/switch.c | 57 +++++++++++++++++++++-
4 files changed, 78 insertions(+), 29 deletions(-)

diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index e5e201314c87..3b56841eb328 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -407,11 +407,11 @@ static inline bool __hyp_handle_ptrauth(struct kvm_vcpu *vcpu)
}

/*
- * Return true when we were able to fixup the guest exit and should return to
- * the guest, false when we should restore the host state and return to the
- * main run loop.
+ * Prologue for the guest fixup, populating ESR_EL2 and fixing up PC
+ * if required.
*/
-static inline bool fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
+static inline void fixup_guest_exit_prologue(struct kvm_vcpu *vcpu,
+ u64 *exit_code)
{
if (ARM_EXCEPTION_CODE(*exit_code) != ARM_EXCEPTION_IRQ)
vcpu->arch.fault.esr_el2 = read_sysreg_el2(SYS_ESR);
@@ -430,7 +430,15 @@ static inline bool fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
if (esr_ec == ESR_ELx_EC_HVC32 || esr_ec == ESR_ELx_EC_HVC64)
write_sysreg_el2(read_sysreg_el2(SYS_ELR) - 4, SYS_ELR);
}
+}

+/*
+ * Return true when we were able to fixup the guest exit and should return to
+ * the guest, false when we should restore the host state and return to the
+ * main run loop.
+ */
+static inline bool fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
+{
/*
* We're using the raw exception code in order to only process
* the trap if no SError is pending. We will come back to the
diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
index 92715fa01e88..1931c8667d52 100644
--- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
+++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
@@ -51,32 +51,12 @@ static inline void __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
ctxt_sys_reg(ctxt, SPSR_EL1) = read_sysreg_el1(SYS_SPSR);
}

-static inline u64 from_hw_pstate(const struct kvm_cpu_context *ctxt)
-{
- u64 reg = read_sysreg_el2(SYS_SPSR);
-
- if (__is_hyp_ctxt(ctxt)) {
- u64 mode = reg & (PSR_MODE_MASK | PSR_MODE32_BIT);
-
- switch (mode) {
- case PSR_MODE_EL1t:
- mode = PSR_MODE_EL2t;
- break;
- case PSR_MODE_EL1h:
- mode = PSR_MODE_EL2h;
- break;
- }
-
- return (reg & ~(PSR_MODE_MASK | PSR_MODE32_BIT)) | mode;
- }
-
- return reg;
-}
-
static inline void __sysreg_save_el2_return_state(struct kvm_cpu_context *ctxt)
{
+ /* On VHE, PSTATE is saved in fixup_guest_exit_vhe() */
+ if (!has_vhe())
+ ctxt->regs.pstate = read_sysreg_el2(SYS_SPSR);
ctxt->regs.pc = read_sysreg_el2(SYS_ELR);
- ctxt->regs.pstate = from_hw_pstate(ctxt);

if (cpus_have_final_cap(ARM64_HAS_RAS_EXTN))
ctxt_sys_reg(ctxt, DISR_EL1) = read_sysreg_s(SYS_VDISR_EL2);
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 07bf5c03631b..fb49757d3446 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -166,6 +166,12 @@ static void __pmu_switch_to_host(struct kvm_cpu_context *host_ctxt)
write_sysreg(pmu->events_host, pmcntenset_el0);
}

+static bool fixup_guest_exit_nvhe(struct kvm_vcpu *vcpu, u64 *exit_code)
+{
+ fixup_guest_exit_prologue(vcpu, exit_code);
+ return fixup_guest_exit(vcpu, exit_code);
+}
+
/* Switch to the guest for legacy non-VHE systems */
int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
{
@@ -219,7 +225,7 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
exit_code = __guest_enter(vcpu);

/* And we're baaack! */
- } while (fixup_guest_exit(vcpu, &exit_code));
+ } while (fixup_guest_exit_nvhe(vcpu, &exit_code));

__sysreg_save_state_nvhe(guest_ctxt);
__sysreg32_save_state(vcpu);
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index d9dc470c7790..4d80596e32a5 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -156,12 +156,60 @@ void deactivate_traps_vhe_put(void)
__deactivate_traps_common();
}

+static bool fixup_guest_exit_vhe(struct kvm_vcpu *vcpu, u64 *exit_code,
+ bool hyp_ctxt)
+{
+ u64 pstate = read_sysreg_el2(SYS_SPSR);
+
+ /*
+ * Sync pstate back as early as possible, so that is_hyp_ctxt()
+ * reflects the exact context. It is otherwise possible to get
+ * confused with a VHE guest and ARMv8.4-NV, such as:
+ *
+ * (1) enter guest in host EL0
+ * (2) guest traps to guest vEL2 (no hypervisor intervention)
+ * (3) guest clears virtual HCR_EL2.TGE (no trap either)
+ * (4) host interrupt fires, exit
+ * (5) is_hyp_ctxt() now says "guest" (pstate.M==EL1 and TGE==0)
+ *
+ * If host preemption occurs, vcpu_load/put() will be very confused.
+ *
+ * Consider this as the prologue before the fixup prologue...
+ */
+
+ if (unlikely(hyp_ctxt)) {
+ u64 mode = pstate & PSR_MODE_MASK;
+
+ switch (mode) {
+ case PSR_MODE_EL1t:
+ mode = PSR_MODE_EL2t;
+ break;
+ case PSR_MODE_EL1h:
+ mode = PSR_MODE_EL2h;
+ break;
+ }
+
+ pstate = (pstate & ~PSR_MODE_MASK) | mode;
+ }
+
+ *vcpu_cpsr(vcpu) = pstate;
+
+ fixup_guest_exit_prologue(vcpu, exit_code);
+
+ if (*exit_code == ARM_EXCEPTION_TRAP) {
+ /* more to come here */
+ }
+
+ return fixup_guest_exit(vcpu, exit_code);
+}
+
/* Switch to the guest for VHE systems running in EL2 */
static int __kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
{
struct kvm_cpu_context *host_ctxt;
struct kvm_cpu_context *guest_ctxt;
u64 exit_code;
+ bool hyp_ctxt;

host_ctxt = &this_cpu_ptr(&kvm_host_data)->host_ctxt;
host_ctxt->__hyp_running_vcpu = vcpu;
@@ -188,12 +236,19 @@ static int __kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
sysreg_restore_guest_state_vhe(guest_ctxt);
__debug_switch_to_guest(vcpu);

+ /*
+ * Being in HYP context or not is an invariant here. If we enter in
+ * a given context, we exit in the same context. We can thus only
+ * sample the context once.
+ */
+ WRITE_ONCE(hyp_ctxt, is_hyp_ctxt(vcpu));
+
do {
/* Jump in the fire! */
exit_code = __guest_enter(vcpu);

/* And we're baaack! */
- } while (fixup_guest_exit(vcpu, &exit_code));
+ } while (fixup_guest_exit_vhe(vcpu, &exit_code, READ_ONCE(hyp_ctxt)));

sysreg_save_guest_state_vhe(guest_ctxt);

--
2.29.2