Re: [PATCH 26/26] KVM: arm64: Parametrize exception entry with a target EL

Marc Zyngier
2020-05-27T09:34:17Z

HI Mark,

On 2020-05-19 11:44, Mark Rutland wrote:
> On Wed, Apr 22, 2020 at 01:00:50PM +0100, Marc Zyngier wrote:
>> We currently assume that an exception is delivered to EL1, always.
>> Once we emulate EL2, this no longer will be the case. To prepare
>> for this, add a target_mode parameter.
>>
>> While we're at it, merge the computing of the target PC and PSTATE in
>> a single function that updates both PC and CPSR after saving their
>> previous values in the corresponding ELR/SPSR. This ensures that they
>> are updated in the correct order (a pretty common source of bugs...).
>>
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>> ---
>> arch/arm64/kvm/inject_fault.c | 75
>> ++++++++++++++++++-----------------
>> 1 file changed, 38 insertions(+), 37 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/inject_fault.c
>> b/arch/arm64/kvm/inject_fault.c
>> index d3ebf8bca4b89..3dbcbc839b9c3 100644
>> --- a/arch/arm64/kvm/inject_fault.c
>> +++ b/arch/arm64/kvm/inject_fault.c
>> @@ -26,28 +26,12 @@ enum exception_type {
>> except_type_serror = 0x180,
>> };
>>
>> -static u64 get_except_vector(struct kvm_vcpu *vcpu, enum
>> exception_type type)
>> -{
>> - u64 exc_offset;
>> -
>> - switch (*vcpu_cpsr(vcpu) & (PSR_MODE_MASK | PSR_MODE32_BIT)) {
>> - case PSR_MODE_EL1t:
>> - exc_offset = CURRENT_EL_SP_EL0_VECTOR;
>> - break;
>> - case PSR_MODE_EL1h:
>> - exc_offset = CURRENT_EL_SP_ELx_VECTOR;
>> - break;
>> - case PSR_MODE_EL0t:
>> - exc_offset = LOWER_EL_AArch64_VECTOR;
>> - break;
>> - default:
>> - exc_offset = LOWER_EL_AArch32_VECTOR;
>> - }
>> -
>> - return vcpu_read_sys_reg(vcpu, VBAR_EL1) + exc_offset + type;
>> -}
>> -
>> /*
>> + * This performs the exception entry at a given EL (@target_mode),
>> stashing PC
>> + * and PSTATE into ELR and SPSR respectively, and compute the new
>> PC/PSTATE.
>> + * The EL passed to this function *must* be a non-secure, privileged
>> mode with
>> + * bit 0 being set (PSTATE.SP == 1).
>> + *
>> * When an exception is taken, most PSTATE fields are left unchanged
>> in the
>> * handler. However, some are explicitly overridden (e.g. M[4:0]).
>> Luckily all
>> * of the inherited bits have the same position in the
>> AArch64/AArch32 SPSR_ELx
>> @@ -59,10 +43,35 @@ static u64 get_except_vector(struct kvm_vcpu
>> *vcpu, enum exception_type type)
>> * Here we manipulate the fields in order of the AArch64 SPSR_ELx
>> layout, from
>> * MSB to LSB.
>> */
>> -static unsigned long get_except64_pstate(struct kvm_vcpu *vcpu)
>> +static void enter_exception(struct kvm_vcpu *vcpu, unsigned long
>> target_mode,
>> + enum exception_type type)
>
> Since this is all for an AArch64 target, could we keep `64` in the
> name,
> e.g enter_exception64? That'd mirror the callers below.
>
>> {
>> - unsigned long sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
>> - unsigned long old, new;
>> + unsigned long sctlr, vbar, old, new, mode;
>> + u64 exc_offset;
>> +
>> + mode = *vcpu_cpsr(vcpu) & (PSR_MODE_MASK | PSR_MODE32_BIT);
>> +
>> + if (mode == target_mode)
>> + exc_offset = CURRENT_EL_SP_ELx_VECTOR;
>> + else if ((mode | 1) == target_mode)
>> + exc_offset = CURRENT_EL_SP_EL0_VECTOR;
>
> It would be nice if we could add a mnemonic for the `1` here, e.g.
> PSR_MODE_SP0 or PSR_MODE_THREAD_BIT.

I've addressed both comments as follows:

diff --git a/arch/arm64/include/asm/ptrace.h
b/arch/arm64/include/asm/ptrace.h
index bf57308fcd63..953b6a1ce549 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -35,6 +35,7 @@
#define GIC_PRIO_PSR_I_SET (1 << 4)

/* Additional SPSR bits not exposed in the UABI */
+#define PSR_MODE_THREAD_BIT (1 << 0)
#define PSR_IL_BIT (1 << 20)

/* AArch32-specific ptrace requests */
diff --git a/arch/arm64/kvm/inject_fault.c
b/arch/arm64/kvm/inject_fault.c
index 3dbcbc839b9c..ebfdfc27b2bd 100644
--- a/arch/arm64/kvm/inject_fault.c
+++ b/arch/arm64/kvm/inject_fault.c
@@ -43,8 +43,8 @@ enum exception_type {
* Here we manipulate the fields in order of the AArch64 SPSR_ELx
layout, from
* MSB to LSB.
*/
-static void enter_exception(struct kvm_vcpu *vcpu, unsigned long
target_mode,
- enum exception_type type)
+static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long
target_mode,
+ enum exception_type type)
{
unsigned long sctlr, vbar, old, new, mode;
u64 exc_offset;
@@ -53,7 +53,7 @@ static void enter_exception(struct kvm_vcpu *vcpu,
unsigned long target_mode,

if (mode == target_mode)
exc_offset = CURRENT_EL_SP_ELx_VECTOR;
- else if ((mode | 1) == target_mode)
+ else if ((mode | PSR_MODE_THREAD_BIT) == target_mode)
exc_offset = CURRENT_EL_SP_EL0_VECTOR;
else if (!(mode & PSR_MODE32_BIT))
exc_offset = LOWER_EL_AArch64_VECTOR;
@@ -126,7 +126,7 @@ static void inject_abt64(struct kvm_vcpu *vcpu, bool
is_iabt, unsigned long addr
bool is_aarch32 = vcpu_mode_is_32bit(vcpu);
u32 esr = 0;

- enter_exception(vcpu, PSR_MODE_EL1h, except_type_sync);
+ enter_exception64(vcpu, PSR_MODE_EL1h, except_type_sync);

vcpu_write_sys_reg(vcpu, addr, FAR_EL1);

@@ -156,7 +156,7 @@ static void inject_undef64(struct kvm_vcpu *vcpu)
{
u32 esr = (ESR_ELx_EC_UNKNOWN << ESR_ELx_EC_SHIFT);

- enter_exception(vcpu, PSR_MODE_EL1h, except_type_sync);
+ enter_exception64(vcpu, PSR_MODE_EL1h, except_type_sync);

/*
* Build an unknown exception, depending on the instruction

Thanks,

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