linux-kernel - Re: [PATCH v4 13/34] KVM: arm64: Enable access to sanitized CPU features at EL2

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Message-ID: <YEsLYX2CjNMXF9TN@google.com>
Date:   Fri, 12 Mar 2021 06:34:09 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     catalin.marinas@....com, maz@...nel.org, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        android-kvm@...gle.com, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org, tabba@...gle.com,
        mark.rutland@....com, dbrazdil@...gle.com, mate.toth-pal@....com,
        seanjc@...gle.com, robh+dt@...nel.org, ardb@...nel.org
Subject: Re: [PATCH v4 13/34] KVM: arm64: Enable access to sanitized CPU
 features at EL2

On Thursday 11 Mar 2021 at 19:36:39 (+0000), Will Deacon wrote:
> On Wed, Mar 10, 2021 at 05:57:30PM +0000, Quentin Perret wrote:
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 066030717a4c..f2d8b479ff74 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1154,6 +1154,18 @@ u64 read_sanitised_ftr_reg(u32 id)
> >  }
> >  EXPORT_SYMBOL_GPL(read_sanitised_ftr_reg);
> >  
> > +int copy_ftr_reg(u32 id, struct arm64_ftr_reg *dst)
> > +{
> > +	struct arm64_ftr_reg *regp = get_arm64_ftr_reg(id);
> > +
> > +	if (!regp)
> > +		return -EINVAL;
> > +
> > +	memcpy(dst, regp, sizeof(*regp));
> 
> Can you not just use struct assignment here?

Sure.

> > diff --git a/arch/arm64/kvm/hyp/nvhe/cache.S b/arch/arm64/kvm/hyp/nvhe/cache.S
> > new file mode 100644
> > index 000000000000..36cef6915428
> > --- /dev/null
> > +++ b/arch/arm64/kvm/hyp/nvhe/cache.S
> > @@ -0,0 +1,13 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * Code copied from arch/arm64/mm/cache.S.
> > + */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/assembler.h>
> > +#include <asm/alternative.h>
> > +
> > +SYM_FUNC_START_PI(__flush_dcache_area)
> > +	dcache_by_line_op civac, sy, x0, x1, x2, x3
> > +	ret
> > +SYM_FUNC_END_PI(__flush_dcache_area)
> 
> Separate patch for this? (or fold it into that one really near the start
> that introduces some other PI helpers).

Right, I guess that'll make reverts and such easier so why not.

> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 4f2f1e3145de..84be93df52fa 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -21,6 +21,7 @@
> >  #include <asm/debug-monitors.h>
> >  #include <asm/esr.h>
> >  #include <asm/kvm_arm.h>
> > +#include <asm/kvm_cpufeature.h>
> >  #include <asm/kvm_emulate.h>
> >  #include <asm/kvm_hyp.h>
> >  #include <asm/kvm_mmu.h>
> > @@ -2775,3 +2776,23 @@ void kvm_sys_reg_table_init(void)
> >  	/* Clear all higher bits. */
> >  	cache_levels &= (1 << (i*3))-1;
> >  }
> > +
> > +#undef KVM_HYP_CPU_FTR_REG
> > +#define KVM_HYP_CPU_FTR_REG(id, name) \
> > +	{ .sys_id = id, .dst = (struct arm64_ftr_reg *)&kvm_nvhe_sym(name) },
> > +struct __ftr_reg_copy_entry {
> > +	u32			sys_id;
> > +	struct arm64_ftr_reg	*dst;
> > +} hyp_ftr_regs[] __initdata = {
> > +	#include <asm/kvm_cpufeature.h>
> > +};
> 
> This looks a bit elaborate to me. Why can't you just spell things out here
> like:
> 
> #define KVM_HYP_CPU_FTR_REG(id, name) \
> 	{ .sys_id = id, .dst = (struct arm64_ftr_reg *)&kvm_nvhe_sym(name) }
> 
> struct __ftr_reg_copy_entry {
> 	u32			sys_id;
> 	struct arm64_ftr_reg	*dst;
> } hyp_ftr_regs[] __initdata = {
> 	KVM_HYP_CPU_FTR_REG(SYS_CTR_EL0, arm64_ftr_reg_ctrel0),
> 	...
> };
> 
> and then have the header file be a normal, guarded header which just
> declares these things? The id parameter to the macro isn't used there.

I just tried to reduce the boilerplate as much as possible -- in the
current form you only need to add additional features to the header
it'll 'just work'.

Thanks,
Quentin