Re: [PATCH v7 11/25] arm64: irqflags: Use ICC_PMR_EL1 for interrupt masking

From: Ard Biesheuvel
Date: Wed Dec 12 2018 - 12:28:05 EST

On Wed, 12 Dec 2018 at 17:48, Julien Thierry <julien.thierry@xxxxxxx> 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@xxxxxxx>
> Suggested-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/efi.h | 5 +-
> arch/arm64/include/asm/irqflags.h | 123 +++++++++++++++++++++++++++++---------
> 2 files changed, 99 insertions(+), 29 deletions(-)
>
> diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
> index 7ed3208..a9d3ebc 100644
> --- a/arch/arm64/include/asm/efi.h
> +++ b/arch/arm64/include/asm/efi.h
> @@ -42,7 +42,10 @@
>
> efi_status_t __efi_rt_asm_wrapper(void *, const char *, ...);
>
> -#define ARCH_EFI_IRQ_FLAGS_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
> +#define ARCH_EFI_IRQ_FLAGS_MASK \
> + (system_uses_irq_prio_masking() ? \
> + GIC_PRIO_IRQON : \
> + (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT))
>

This mask is used to determine whether we return from a firmware call
with a different value for the I flag than we entered it with. So
instead of changing the mask, we should change the way we record DAIF,
given that the firmware is still going to poke the I bit if it
misbehaves, regardless of whether the OS happens to use priorities for
interrupt masking.

It also means that the NMI concept is a best effort thing only, given
that uncooperative firmware could prevent them from being delivered.



> /* arch specific definitions used by the stub code */
>
> 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 @@
> /*
> * 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");
> }
>
> /*
> + * 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; \
> +})
> +
> +/*
> * 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)
> :
> : "memory");
> +
> + return _get_irqflags(daif_bits, pmr);
> +}
> +
> +#undef _get_irqflags
> +
> +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 +136,32 @@ 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
>