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SubjectRe: [PATCH v8 12/26] arm64: irqflags: Use ICC_PMR_EL1 for interrupt masking
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On 18/01/2019 16:35, Dave Martin wrote:
> On Tue, Jan 08, 2019 at 02:07:30PM +0000, Julien Thierry 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 | 11 ++++
>> arch/arm64/include/asm/irqflags.h | 123 +++++++++++++++++++++++++++++---------
>> 2 files changed, 106 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
>> index 7ed3208..134ff6e 100644
>> --- a/arch/arm64/include/asm/efi.h
>> +++ b/arch/arm64/include/asm/efi.h
>> @@ -44,6 +44,17 @@
>>
>> #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) \
>> + do { \
>> + (state_flags) = read_sysreg(daif); \
>> + } while (0)
>> +
>> +#define arch_efi_restore_flags(state_flags) \
>> + do { \
>> + write_sysreg(state_flags, daif); \
>> + } while (0)
>> +
>> +
>
> Randomly commenting a few minor nits as I glance down my mailbox...
>
> There's no need to protect single statements with do { } while(0).
>
> Just protect an expression statement that could be misparsed with ( ).
>
> ->
>
> #define arch_efi_save_flags(state_flags) ((state_flags) = read_sysreg(daif))

For the efi_save_flags(), I wanted to avoid it getting used as an
expression.

Would casting the assignment expression to (void) be acceptable?

> #define arch_efi_restore_flags(state_flags) write_sysreg(state_flags, daif)

For this one, write_sysreg() is already a statement, so yes, I
definitely don't need a do { } while (0) here.

>
> [...]
>
>> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
>> index 24692ed..fa3b06f 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
>> @@ -36,47 +38,96 @@
>
> [...]
>
>> /*
>> + * Having two ways to control interrupt status is a bit complicated. Some
>> + * locations like exception entries will have PSR.I bit set by the architecture
>> + * while PMR is unmasked.
>> + * We need the irqflags to represent that interrupts are disabled in such cases.
>> + *
>> + * For this, we lower the value read from PMR when the I bit is set so it is
>> + * considered as an irq masking priority. (With PMR, lower value means masking
>> + * more interrupts).
>> + */
>> +#define _get_irqflags(daif_bits, pmr) \
>> +({ \
>> + unsigned long flags; \
>> + \
>> + BUILD_BUG_ON(GIC_PRIO_IRQOFF < (GIC_PRIO_IRQON & ~PSR_I_BIT)); \
>> + asm volatile(ALTERNATIVE( \
>> + "mov %0, %1\n" \
>> + "nop\n" \
>> + "nop", \
>> + "and %0, %1, #" __stringify(PSR_I_BIT) "\n" \
>> + "mvn %0, %0\n" \
>> + "and %0, %0, %2", \
>> + ARM64_HAS_IRQ_PRIO_MASKING) \
>> + : "=&r" (flags) \
>> + : "r" (daif_bits), "r" (pmr) \
>> + : "memory"); \
>> + \
>> + flags; \
>> +})
>
> Nit: does this need to be a macro?
>
> ({ ... }) is mildly gross and it's preferable to avoid it if the code
> works just as well without...
>
> pmr would need to be passed as a pointer, with "r" (*pmr) in the asm,
> but I think it would compile down to precisely the same code.
>

The only motivation for it to be a macro was to be able to #undef it
after its use.

But with Catalin's suggestion, looks like we can makes things simple and
avoid having a separate macro/function.

>> +
>> +/*
>> * Save the current interrupt enable state.
>> */
>> static inline unsigned long arch_local_save_flags(void)
>> {
>> - unsigned long flags;
>> - asm volatile(
>> - "mrs %0, daif // arch_local_save_flags"
>> - : "=r" (flags)
>> + unsigned long daif_bits;
>> + unsigned long pmr; // Only used if alternative is on
>> +
>> + daif_bits = read_sysreg(daif);
>> +
>> + // Get PMR
>> + asm volatile(ALTERNATIVE(
>> + "nop",
>> + "mrs_s %0, " __stringify(SYS_ICC_PMR_EL1),
>> + ARM64_HAS_IRQ_PRIO_MASKING)
>> + : "=&r" (pmr)
>
> Why earlyclobber?
>>> :
>> : "memory");
>
> [...]
>
>> @@ -85,16 +136,32 @@ static inline unsigned long arch_local_save_flags(void)
>
> [...]
>
>> 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)
>
> Why earlyclobber? %0 is not written before the reading of any input
> argument so far as I can see, in either alternative.
>

I didn't really understand what the earlyclobber semantic was, thanks
for explaining it.

Thanks,

--
Julien Thierry

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