Re: [PATCH V9 00/10] arm64/perf: Enable branch stack sampling

[Mark Brown]

On Thu, Mar 23, 2023 at 09:55:47AM +0530, Anshuman Khandual wrote:
> On 3/22/23 00:32, Mark Brown wrote:

> > document a requirement for those traps to be disabled now in case we
> > need them later, and do so during EL2 setup for KVM guests?  That could
> > always be done incrementally.

> Unlike all other instruction trap enable fields in SYS_HFGITR_EL2, these BRBE
> instructions ones are actually inverted in semantics i.e the particular fields
> need to be set for these traps to be disabled in EL2.

Right, for backwards compatibility all newly added fields are trap by
default.

> SYS_HFGITR_EL2.nBRBIALL
> SYS_HFGITR_EL2.nBRBINJ

> By default entire SYS_HFGITR_EL2 is set as cleared during init and that would
> prevent a guest from using BRBE.

It should prevent the host as well shouldn't it?  

> I guess something like the following (untested) needs to be done, to enable
> BRBE in guests.

> +       mrs     x1, id_aa64dfr0_el1
> +       ubfx    x1, x1, #ID_AA64DFR0_EL1_BRBE_SHIFT, #4
> +       cbz     x1, .Lskip_brbe_\@
> +       mov     x0, xzr
> +       orr     x0, x0, #HFGITR_EL2_nBRBIALL
> +       orr     x0, x0, #HFGITR_EL2_nBRBINJ
> +       msr_s   SYS_HFGITR_EL2, x0
> +
> +.Lskip_brbe_\@:

Yes, looks roughly what I'd expect.

> > I've got a patch adding the definition of that register to sysreg which
> > I should be sending shortly, no need to duplicate that effort.

> Sure, I assume you are moving the existing definition for SYS_HFGITR_EL2 along
> with all its fields from ../include/asm/sysreg.h to ../tools/sysreg. Right, it
> makes sense.

No fields at the minute but yes, like the other conversions.

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