Re: [PATCH V3 02/16] arm64/cpufeature: Drop TraceFilt feature exposure from ID_DFR0 register

[Will Deacon <will@kernel.org>]

On Tue, May 05, 2020 at 12:20:41PM +0530, Anshuman Khandual wrote:
> On 05/05/2020 01:54 AM, Will Deacon wrote:
> > On Sat, May 02, 2020 at 07:03:51PM +0530, Anshuman Khandual wrote:
> >> ID_DFR0 based TraceFilt feature should not be exposed to guests. Hence lets
> >> drop it.
> >>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Marc Zyngier <maz@kernel.org>
> >> Cc: Mark Rutland <mark.rutland@arm.com>
> >> Cc: James Morse <james.morse@arm.com>
> >> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> >> Cc: linux-arm-kernel@lists.infradead.org
> >> Cc: linux-kernel@vger.kernel.org
> >>
> >> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> >> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> >> ---
> >>  arch/arm64/kernel/cpufeature.c | 1 -
> >>  1 file changed, 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> >> index 6d032fbe416f..51386dade423 100644
> >> --- a/arch/arm64/kernel/cpufeature.c
> >> +++ b/arch/arm64/kernel/cpufeature.c
> >> @@ -435,7 +435,6 @@ static const struct arm64_ftr_bits ftr_id_pfr1[] = {
> >>  };
> >>  
> >>  static const struct arm64_ftr_bits ftr_id_dfr0[] = {
> >> -	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 28, 4, 0),
> > 
> > Hmm, this still confuses me. Is this not now FTR_NONSTRICT? Why is that ok?
> 
> Mark had mentioned about it earlier (https://patchwork.kernel.org/patch/11287805/)
> Did I misinterpret the first part ? Could not figure "capping the emulated debug
> features" part. Probably, Mark could give some more details.
> 
> From the earlier discussion:
> 
> * ID_DFR0 fields need more thought; we should limit what we expose here.
>   I don't think it's valid for us to expose TraceFilt, and I suspect we
>   need to add capping for debug features we currently emulate.

Sorry, I for confused (again) by the cpufeature code :) I'm going to add
the following to my comment:


diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c1d44d127baa..9b05843d67af 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -53,6 +53,11 @@
  *   arbitrary physical CPUs, but some features not present on the host are
  *   also advertised and emulated. Look at sys_reg_descs[] for the gory
  *   details.
+ *
+ * - If the arm64_ftr_bits[] for a register has a missing field, then this
+ *   field is treated as STRICT RES0, including for read_sanitised_ftr_reg().
+ *   This is stronger than FTR_HIDDEN and can be used to hide features from
+ *   KVM guests.
  */
 
 #define pr_fmt(fmt) "CPU features: " fmt


However, I think we really want to get rid of ftr_generic_32bits[] entirely
and spell out all of the register fields, even just using comments for the
fields we're omitting:


@@ -425,7 +430,7 @@ static const struct arm64_ftr_bits ftr_id_pfr1[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_dfr0[] = {
-	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 28, 4, 0),
+	/* 31:28	TraceFilt */
 	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 24, 4, 0xf),	/* PerfMon */
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 20, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 16, 4, 0),


Longer term, I think we'll probably want to handle these within
ARM64_FTR_BITS, as we may end up with features that we want to hide from
KVM guests but not from the host kernel.

Will