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SubjectRe: [PATCH v2 11/20] arm64: capabilities: Add support for features enabled early
On Wed, Feb 07, 2018 at 06:34:37PM +0000, Suzuki K Poulose wrote:
> On 07/02/18 10:38, Dave Martin wrote:
> >On Wed, Jan 31, 2018 at 06:27:58PM +0000, Suzuki K Poulose wrote:
> >>The kernel detects and uses some of the features based on the boot
> >>CPU and expects that all the following CPUs conform to it. e.g,
> >>with VHE and the boot CPU running at EL2, the kernel decides to
> >>keep the kernel running at EL2. If another CPU is brought up without
> >>this capability, we use custom hooks (via check_early_cpu_features())
> >>to handle it. To handle such capabilities add support for detecting
> >>and enabling capabilities based on the boot CPU.
> >>
> >>A bit is added to indicate if the capability should be detected
> >>early on the boot CPU. The infrastructure then ensures that such
> >>capabilities are probed and "enabled" early on in the boot CPU
> >>and, enabled on the subsequent CPUs.
> >>
> >>Cc: Julien Thierry <julien.thierry@arm.com>
> >>Cc: Dave Martin <dave.martin@arm.com>
> >>Cc: Will Deacon <will.deacon@arm.com>
> >>Cc: Mark Rutland <mark.rutland@arm.com>
> >>Cc: Marc Zyngier <marc.zyngier@arm.com>
> >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> >>---
> >> arch/arm64/include/asm/cpufeature.h | 48 +++++++++++++++++++++++++++++--------
> >> arch/arm64/kernel/cpufeature.c | 48 +++++++++++++++++++++++++++----------
> >> 2 files changed, 74 insertions(+), 22 deletions(-)
> >>
> >>diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h

[...]

> >> * 3) Verification: When a CPU is brought online (e.g, by user or by the kernel),
> >> * the kernel should make sure that it is safe to use the CPU, by verifying
> >>@@ -139,11 +148,22 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
> >> *
> >> * As explained in (2) above, capabilities could be finalised at different
> >> * points in the execution. Each CPU is verified against the "finalised"
> >>- * capabilities and if there is a conflict, the kernel takes an action, based
> >>- * on the severity (e.g, a CPU could be prevented from booting or cause a
> >>- * kernel panic). The CPU is allowed to "affect" the state of the capability,
> >>- * if it has not been finalised already. See section 5 for more details on
> >>- * conflicts.
> >>+ * capabilities.
> >>+ *
> >>+ * x------------------------------------------------------------------- x
> >>+ * | Verification: | Boot CPU | SMP CPUs by kernel | CPUs by user |
> >>+ * |--------------------------------------------------------------------|
> >>+ * | Primary boot CPU | | | |
> >>+ * | capability | n | y | y |
> >>+ * |--------------------------------------------------------------------|
> >>+ * | All others | n | n | y |
> >>+ * x--------------------------------------------------------------------x
> >
> >Minor clarify nit: it's not obvious that "n" means "no conflict" and "y"
> >means "conflict".
> >
> >Could we have blank cell versus "X" (with a note saying what that
> >means), or "ok" versus "CONFLICT"?
>
> This is not strictly about conflicts, but about what each CPU get
> verified against. Since there are multiple stages of "finalisation"

You're right: I meant something like "potential conflict", but I hadn't
read the previous paragraph carefully enough and didn't explain what I
meant very well.

> for the capabilities, the table shows how the CPUs get verified.
>
> Would it help if I changed the description above the table to :
>
> * As explained in (2) above, capabilities could be finalised at different
> * points in the execution. Each CPU is verified against the "finalised"
> * capabilities. The following table shows, the capabilities verified
> * against each CPU in the system.
> *
> * x------------------------------------------------------------------- x
> * | Verified against: | Boot CPU | SMP CPUs by kernel | CPUs by user |

I still find it a bit cryptic.

Would it be simpler just to write this out in prose, with reference to
the actual capability types? I feel that things have to be abbreviated
a bit to fit nicely into the table otherwise.

What about:

* As explained in (2) above, capabilities could be finalised at different
* points in the execution, depending on the capability type. Each newly booted
* CPU is verified against those capabilities that have been finalised by the
* time that CPU boots:
*
* * SCOPE_BOOT_CPU: all CPUs are verified against the capability except
* for the primary boot CPU.
*
* * SCOPE_LOCAL_CPU, SCOPE_SYSTEM: all CPUs hotplugged on by the user
* after kernel boot are verified against the capability.
> >>+ * If there is a conflict, the kernel takes an action, based on the severity
> >>+ * (e.g, a CPU could be prevented from booting or cause a kernel panic).
> >>+ * The CPU is allowed to "affect" the state of the capability, if it has not
> >>+ * been finalised already. See section 5 for more details on conflicts.
> >> *
> >> * 4) Action: As mentioned in (2), the kernel can take an action for each detected
> >> * capability, on all CPUs on the system. This is always initiated only after

[...]

Cheers
---Dave
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