Re: [PATCH v9 12/26] arm64: irqflags: Use ICC_PMR_EL1 for interrupt masking

[Julien Thierry <julien.thierry@arm.com>]

On 21/01/2019 15:45, Ard Biesheuvel wrote:
> On Mon, 21 Jan 2019 at 16:34, Julien Thierry <julien.thierry@arm.com> wrote:
>>
>> Instead disabling interrupts by setting the PSR.I bit, use a priority
>> higher than the one used for interrupts to mask them via PMR.
>>
>> When using PMR to disable interrupts, the value of PMR will be used
>> instead of PSR.[DAIF] for the irqflags.
>>
>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
>> Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Cc: Oleg Nesterov <oleg@redhat.com>
>> ---
>>  arch/arm64/include/asm/efi.h      |   6 +++
>>  arch/arm64/include/asm/irqflags.h | 104 ++++++++++++++++++++++++++++----------
>>  2 files changed, 82 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
>> index 7ed3208..3db6772 100644
>> --- a/arch/arm64/include/asm/efi.h
>> +++ b/arch/arm64/include/asm/efi.h
>> @@ -44,6 +44,12 @@
>>
>>  #define ARCH_EFI_IRQ_FLAGS_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
>>
>> +#define arch_efi_save_flags(state_flags)               \
>> +       ((void)((state_flags) = read_sysreg(daif)))
>> +
>> +#define arch_efi_restore_flags(state_flags)    write_sysreg(state_flags, daif)
>> +
>> +
>>  /* arch specific definitions used by the stub code */
>>
>>  /*
> 
> For the EFI changes
> 
> Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Thanks!

> 
> although it deserves a comment as to why it is unaffected by
> ARM64_HAS_IRQ_PRIO_MASKING

So far we've relied on EFI not modifying PMR, otherwise we could've
ended up without any interrupts in Linux after coming back from an EFI
service.

Now, the ARM64_HAS_IRQ_PRIO_MASKING only modifies the behaviour of
Linux, but we should have the same expectations from EFI. If we want to
be paranoid we could save/restore PMR along with the DAIF flags.

But you're right, it is fair to add a comment clearly stating that we
expect EFI not to modify PMR. I'll include that in the next posting.

Thanks,

Julien

> 
> 
>> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
>> index 24692ed..7e82a92 100644
>> --- a/arch/arm64/include/asm/irqflags.h
>> +++ b/arch/arm64/include/asm/irqflags.h
>> @@ -18,7 +18,9 @@
>>
>>  #ifdef __KERNEL__
>>
>> +#include <asm/alternative.h>
>>  #include <asm/ptrace.h>
>> +#include <asm/sysreg.h>
>>
>>  /*
>>   * Aarch64 has flags for masking: Debug, Asynchronous (serror), Interrupts and
>> @@ -36,33 +38,31 @@
>>  /*
>>   * CPU interrupt mask handling.
>>   */
>> -static inline unsigned long arch_local_irq_save(void)
>> -{
>> -       unsigned long flags;
>> -       asm volatile(
>> -               "mrs    %0, daif                // arch_local_irq_save\n"
>> -               "msr    daifset, #2"
>> -               : "=r" (flags)
>> -               :
>> -               : "memory");
>> -       return flags;
>> -}
>> -
>>  static inline void arch_local_irq_enable(void)
>>  {
>> -       asm volatile(
>> -               "msr    daifclr, #2             // arch_local_irq_enable"
>> -               :
>> +       unsigned long unmasked = GIC_PRIO_IRQON;
>> +
>> +       asm volatile(ALTERNATIVE(
>> +               "msr    daifclr, #2             // arch_local_irq_enable\n"
>> +               "nop",
>> +               "msr_s  " __stringify(SYS_ICC_PMR_EL1) ",%0\n"
>> +               "dsb    sy",
>> +               ARM64_HAS_IRQ_PRIO_MASKING)
>>                 :
>> +               : "r" (unmasked)
>>                 : "memory");
>>  }
>>
>>  static inline void arch_local_irq_disable(void)
>>  {
>> -       asm volatile(
>> -               "msr    daifset, #2             // arch_local_irq_disable"
>> -               :
>> +       unsigned long masked = GIC_PRIO_IRQOFF;
>> +
>> +       asm volatile(ALTERNATIVE(
>> +               "msr    daifset, #2             // arch_local_irq_disable",
>> +               "msr_s  " __stringify(SYS_ICC_PMR_EL1) ", %0",
>> +               ARM64_HAS_IRQ_PRIO_MASKING)
>>                 :
>> +               : "r" (masked)
>>                 : "memory");
>>  }
>>
>> @@ -71,12 +71,44 @@ static inline void arch_local_irq_disable(void)
>>   */
>>  static inline unsigned long arch_local_save_flags(void)
>>  {
>> +       unsigned long daif_bits;
>>         unsigned long flags;
>> -       asm volatile(
>> -               "mrs    %0, daif                // arch_local_save_flags"
>> -               : "=r" (flags)
>> -               :
>> +
>> +       daif_bits = read_sysreg(daif);
>> +
>> +       /*
>> +        * The asm is logically equivalent to:
>> +        *
>> +        * if (system_uses_irq_prio_masking())
>> +        *      flags = (daif_bits & PSR_I_BIT) ?
>> +        *                      GIC_PRIO_IRQOFF :
>> +        *                      read_sysreg_s(SYS_ICC_PMR_EL1);
>> +        * else
>> +        *      flags = daif_bits;
>> +        */
>> +       asm volatile(ALTERNATIVE(
>> +                       "mov    %0, %1\n"
>> +                       "nop\n"
>> +                       "nop",
>> +                       "mrs_s  %0, " __stringify(SYS_ICC_PMR_EL1) "\n"
>> +                       "ands   %1, %1, " __stringify(PSR_I_BIT) "\n"
>> +                       "csel   %0, %0, %2, eq",
>> +                       ARM64_HAS_IRQ_PRIO_MASKING)
>> +               : "=&r" (flags), "+r" (daif_bits)
>> +               : "r" (GIC_PRIO_IRQOFF)
>>                 : "memory");
>> +
>> +       return flags;
>> +}
>> +
>> +static inline unsigned long arch_local_irq_save(void)
>> +{
>> +       unsigned long flags;
>> +
>> +       flags = arch_local_save_flags();
>> +
>> +       arch_local_irq_disable();
>> +
>>         return flags;
>>  }
>>
>> @@ -85,16 +117,32 @@ static inline unsigned long arch_local_save_flags(void)
>>   */
>>  static inline void arch_local_irq_restore(unsigned long flags)
>>  {
>> -       asm volatile(
>> -               "msr    daif, %0                // arch_local_irq_restore"
>> -       :
>> -       : "r" (flags)
>> -       : "memory");
>> +       asm volatile(ALTERNATIVE(
>> +                       "msr    daif, %0\n"
>> +                       "nop",
>> +                       "msr_s  " __stringify(SYS_ICC_PMR_EL1) ", %0\n"
>> +                       "dsb    sy",
>> +                       ARM64_HAS_IRQ_PRIO_MASKING)
>> +               : "+r" (flags)
>> +               :
>> +               : "memory");
>>  }
>>
>>  static inline int arch_irqs_disabled_flags(unsigned long flags)
>>  {
>> -       return flags & PSR_I_BIT;
>> +       int res;
>> +
>> +       asm volatile(ALTERNATIVE(
>> +                       "and    %w0, %w1, #" __stringify(PSR_I_BIT) "\n"
>> +                       "nop",
>> +                       "cmp    %w1, #" __stringify(GIC_PRIO_IRQOFF) "\n"
>> +                       "cset   %w0, ls",
>> +                       ARM64_HAS_IRQ_PRIO_MASKING)
>> +               : "=&r" (res)
>> +               : "r" ((int) flags)
>> +               : "memory");
>> +
>> +       return res;
>>  }
>>  #endif
>>  #endif
>> --
>> 1.9.1
>>

-- 
Julien Thierry