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

From: Marc Zyngier <maz@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	Andre Przywara <andre.przywara@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Jintack Lim <jintack@cs.columbia.edu>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	George Cherian <gcherian@marvell.com>,
	"Zengtao (B)" <prime.zeng@hisilicon.com>,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>
Subject: Re: [PATCH 26/26] KVM: arm64: Parametrize exception entry with a target EL
Date: Wed, 27 May 2020 10:34:09 +0100	[thread overview]
Message-ID: <db34b0fbd58275a0a2a0c9108b9507d6@kernel.org> (raw)
In-Reply-To: <20200519104457.GA19548@C02TD0UTHF1T.local>

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...
