Hi Suzuki

-----Original Message-----
From: Catalin Marinas <catalin.marinas@arm.com>
Sent: 2019年9月19日 0:46
To: Suzuki Poulose <Suzuki.Poulose@arm.com>
Cc: Justin He (Arm Technology China) <redacted>; Will Deacon
[off-list ref]; Mark Rutland [off-list ref]; James Morse
[off-list ref]; Marc Zyngier [off-list ref]; Matthew
Wilcox [off-list ref]; Kirill A. Shutemov
[off-list ref]; linux-arm-kernel@lists.infradead.org;
linux-kernel@vger.kernel.org; linux-mm@kvack.org; Punit Agrawal
[off-list ref]; Anshuman Khandual
[off-list ref]; Jun Yao [off-list ref];
Alex Van Brunt [off-list ref]; Robin Murphy
[off-list ref]; Thomas Gleixner [off-list ref];
Andrew Morton [off-list ref]; Jérôme Glisse
[off-list ref]; Ralph Campbell [off-list ref];
hejianet@gmail.com; Kaly Xin (Arm Technology China) [off-list ref]
Subject: Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper
cpu_has_hw_af()

On Wed, Sep 18, 2019 at 03:20:41PM +0100, Suzuki K Poulose wrote:

quoted

On 18/09/2019 14:19, Jia He wrote:

quoted

diff --git a/arch/arm64/include/asm/cpufeature.h

b/arch/arm64/include/asm/cpufeature.h

quoted

quoted

index c96ffa4722d3..206b6e3954cf 100644

--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h

@@ -390,6 +390,7 @@ extern DECLARE_BITMAP(boot_capabilities,

ARM64_NPATCHABLE);

quoted

quoted

          for_each_set_bit(cap, cpu_hwcaps, ARM64_NCAPS)
  bool this_cpu_has_cap(unsigned int cap);
+bool cpu_has_hw_af(void);
  void cpu_set_feature(unsigned int num);
  bool cpu_have_feature(unsigned int num);
  unsigned long cpu_get_elf_hwcap(void);

diff --git a/arch/arm64/kernel/cpufeature.c

b/arch/arm64/kernel/cpufeature.c

quoted

quoted

index b1fdc486aed8..c5097f58649d 100644

--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -1141,6 +1141,12 @@ static bool has_hw_dbm(const struct

arm64_cpu_capabilities *cap,

quoted

quoted

          return true;
  }
+/* Decouple AF from AFDBM. */
+bool cpu_has_hw_af(void)
+{

Sorry for not having asked this earlier. Are we interested in,

"whether *this* CPU has AF support ?" or "whether *at least one*
CPU has the AF support" ? The following code does the former.

quoted

+ return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf);

In a non-preemptible context, the former is ok (per-CPU).

Yes, just as what Catalin explained, we need the former because the
pagefault occurred in every cpus

--
Cheers,
Justin (Jia He)

--
Catalin

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