linux-kernel - Re: [PATCH v2 09/15] arm64: KVM: add support to save/restore SPE profiling buffer controls

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Date:   Wed, 29 May 2019 09:26:35 +0100
From:   Julien Thierry <julien.thierry@....com>
To:     Sudeep Holla <sudeep.holla@....com>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v2 09/15] arm64: KVM: add support to save/restore SPE
 profiling buffer controls

Hi Sudeep,

On 05/23/2019 11:34 AM, Sudeep Holla wrote:
> Currently since we don't support profiling using SPE in the guests,
> we just save the PMSCR_EL1, flush the profiling buffers and disable
> sampling. However in order to support simultaneous sampling both in
> the host and guests, we need to save and reatore the complete SPE
> profiling buffer controls' context.
> 
> Let's add the support for the same and keep it disabled for now.
> We can enable it conditionally only if guests are allowed to use
> SPE.
> 
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> ---
>  arch/arm64/kvm/hyp/debug-sr.c | 44 ++++++++++++++++++++++++++++-------
>  1 file changed, 35 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> index a2714a5eb3e9..a4e6eaf5934f 100644
> --- a/arch/arm64/kvm/hyp/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/debug-sr.c
> @@ -66,7 +66,8 @@
>  	default:	write_debug(ptr[0], reg, 0);			\
>  	}
>  
> -static void __hyp_text __debug_save_spe_nvhe(struct kvm_cpu_context *ctxt)
> +static void __hyp_text
> +__debug_save_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)

Rather that add a boolean to just indicate "do more stuff" I'd suggest
having two separate functions.

Also this would be an opportunity to fix the naming of this function
which doesn't just save sve context, it also flushes the context and
disables it.

So maybe have a: void __debug_spe_flush_ctx(struct kvm_cpu_context *ctx);

Maybe adapt the name to make it understandable that it does save PMSCR.

and void __debug_spe_save_ctx(struct kvm_cpu_context *ctx);

Which would save the registers you save under the full_ctx condition.

Cheers,

Julien