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Date: Tue, 9 Oct 2018 16:56:16 +0200
From: "Edgar E. Iglesias" <edgar.iglesias@xilinx.com>
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Subject: Re: [Qemu-devel] [PATCH v1 10/12] target/arm: Add the Cortex-A72
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To: Laurent Desnogues <laurent.desnogues@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, figlesia@xilinx.com, sstabellini@kernel.org, sai.pavan.boddu@xilinx.com, frasse.iglesias@gmail.com, alistair@alistair23.me, Richard Henderson <richard.henderson@linaro.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, frederic.konrad@adacore.com, qemu-arm <qemu-arm@nongnu.org>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>

On Tue, Oct 09, 2018 at 03:40:13PM +0200, Laurent Desnogues wrote:
> Hello,
> 
> On Tue, Oct 9, 2018 at 3:19 PM Edgar E. Iglesias
> <edgar.iglesias@xilinx.com> wrote:
> >
> > Another A72 related thing I wanted to check with you. A month or two ago I was
> > looking at an issue with Linux running very slowly on our models.
> > Something that popped up was that Linux was running a couple of spectre related
> > "workarounds" and "hardening" sequences on the QEMU A72s.
> >
> > There are a couple of bits in the ID_AARCH64_PFR0 register that
> > Linux checks before enabling the sequences but I never found any
> > documentation of them in the specs. Bits 56 and 60.
> >
> > In Linux these are refered to as:
> > ID_AA64PFR0_CSV2_SHIFT
> > ID_AA64PFR0_CSV3_SHIFT
> >
> > This is what we have in our tree:
> >
> >     cpu->gic_vprebits = 5;
> >     define_arm_cp_regs(cpu, cortex_a57_a53_cp_reginfo);
> >
> >     /* Xilinx FIXUPs.  */
> >     /* These indicate the BP hardening and KPTI aren't needed.  */
> >     cpu->id_aa64pfr0 |= (uint64_t)1 << 56; /* BP.  */
> >     cpu->id_aa64pfr0 |= (uint64_t)1 << 60; /* KPTI.  */
> > }
> >
> > Do you know what these are?
> > Should we be setting these in QEMU?
> 
> These fields are publicly documented in the system register specification:
> 
> https://developer.arm.com/products/architecture/cpu-architecture/a-profile/exploration-tools
> 
> These are ARMv8.5 fields, I don't think these should be set by default
> for Cortex-A72. Of course nothing prevents you from defining a
> specific CPU with these fields set to boot faster :-)  Or perhaps add
> a property to override the default value of these registers?

Thanks, I had no idea these documents existed!

Cheers,
Edgar