Subject: Re: [PATCH v5 36/69] KVM: arm64: nv: Filter out unsupported features from ID regs

From: Marc Zyngier <[email-masked]>
Date: Tue, 21 Dec 2021 09:10:09 +0000

On Tue, 21 Dec 2021 06:03:49 +0000,
Ganapatrao Kulkarni <[email-masked]> wrote:
> 
> 
> 
> On 20-12-2021 03:26 pm, Marc Zyngier wrote:
> > On Mon, 20 Dec 2021 07:26:50 +0000,
> > Ganapatrao Kulkarni <[email-masked]> wrote:
> >> 
> >> 
> >> Hi Marc,
> >> 
> >> On 30-11-2021 01:31 am, Marc Zyngier wrote:
> >>> As there is a number of features that we either can't support,
> >>> or don't want to support right away with NV, let's add some
> >>> basic filtering so that we don't advertize silly things to the
> >>> EL2 guest.
> >>> 
> >>> Whilst we are at it, avertize ARMv8.4-TTL as well as ARMv8.5-GTG.
> >>> 
> >>> Signed-off-by: Marc Zyngier <[email-masked]>
> >>> ---
> >>>    arch/arm64/include/asm/kvm_nested.h |   6 ++
> >>>    arch/arm64/kvm/nested.c             | 152 ++++++++++++++++++++++++++++
> >>>    arch/arm64/kvm/sys_regs.c           |   4 +-
> >>>    arch/arm64/kvm/sys_regs.h           |   2 +
> >>>    4 files changed, 163 insertions(+), 1 deletion(-)
> >>> 
> >>> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> >>> index 07c15f51cf86..026ddaad972c 100644
> >>> --- a/arch/arm64/include/asm/kvm_nested.h
> >>> +++ b/arch/arm64/include/asm/kvm_nested.h
> >>> @@ -67,4 +67,10 @@ extern bool __forward_traps(struct kvm_vcpu *vcpu, unsigned int reg,
> >>>    extern bool forward_traps(struct kvm_vcpu *vcpu, u64 control_bit);
> >>>    extern bool forward_nv_traps(struct kvm_vcpu *vcpu);
> >>>    +struct sys_reg_params;
> >>> +struct sys_reg_desc;
> >>> +
> >>> +void access_nested_id_reg(struct kvm_vcpu *v, struct sys_reg_params *p,
> >>> +			  const struct sys_reg_desc *r);
> >>> +
> >>>    #endif /* __ARM64_KVM_NESTED_H */
> >>> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> >>> index 42a96c8d2adc..19b674983e13 100644
> >>> --- a/arch/arm64/kvm/nested.c
> >>> +++ b/arch/arm64/kvm/nested.c
> >>> @@ -20,6 +20,10 @@
> >>>    #include <linux/kvm_host.h>
> >>>      #include <asm/kvm_emulate.h>
> >>> +#include <asm/kvm_nested.h>
> >>> +#include <asm/sysreg.h>
> >>> +
> >>> +#include "sys_regs.h"
> >>>      /*
> >>>     * Inject wfx to the virtual EL2 if this is not from the virtual EL2 and
> >>> @@ -38,3 +42,151 @@ int handle_wfx_nested(struct kvm_vcpu *vcpu, bool is_wfe)
> >>>      	return -EINVAL;
> >>>    }
> >>> +
> >>> +/*
> >>> + * Our emulated CPU doesn't support all the possible features. For the
> >>> + * sake of simplicity (and probably mental sanity), wipe out a number
> >>> + * of feature bits we don't intend to support for the time being.
> >>> + * This list should get updated as new features get added to the NV
> >>> + * support, and new extension to the architecture.
> >>> + */
> >>> +void access_nested_id_reg(struct kvm_vcpu *v, struct sys_reg_params *p,
> >>> +			  const struct sys_reg_desc *r)
> >>> +{
> >>> +	u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> >>> +			 (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> >>> +	u64 val, tmp;
> >>> +
> >>> +	if (!nested_virt_in_use(v))
> >>> +		return;
> >>> +
> >>> +	val = p->regval;
> >>> +
> >>> +	switch (id) {
> >>> +	case SYS_ID_AA64ISAR0_EL1:
> >>> +		/* Support everything but O.S. and Range TLBIs */
> >>> +		val &= ~(FEATURE(ID_AA64ISAR0_TLB)	|
> >>> +			 GENMASK_ULL(27, 24)		|
> >>> +			 GENMASK_ULL(3, 0));
> >>> +		break;
> >>> +
> >>> +	case SYS_ID_AA64ISAR1_EL1:
> >>> +		/* Support everything but PtrAuth and Spec Invalidation */
> >>> +		val &= ~(GENMASK_ULL(63, 56)		|
> >>> +			 FEATURE(ID_AA64ISAR1_SPECRES)	|
> >>> +			 FEATURE(ID_AA64ISAR1_GPI)	|
> >>> +			 FEATURE(ID_AA64ISAR1_GPA)	|
> >>> +			 FEATURE(ID_AA64ISAR1_API)	|
> >>> +			 FEATURE(ID_AA64ISAR1_APA));
> >>> +		break;
> >>> +
> >>> +	case SYS_ID_AA64PFR0_EL1:
> >>> +		/* No AMU, MPAM, S-EL2, RAS or SVE */
> >>> +		val &= ~(GENMASK_ULL(55, 52)		|
> >>> +			 FEATURE(ID_AA64PFR0_AMU)	|
> >>> +			 FEATURE(ID_AA64PFR0_MPAM)	|
> >>> +			 FEATURE(ID_AA64PFR0_SEL2)	|
> >>> +			 FEATURE(ID_AA64PFR0_RAS)	|
> >>> +			 FEATURE(ID_AA64PFR0_SVE)	|
> >>> +			 FEATURE(ID_AA64PFR0_EL3)	|
> >>> +			 FEATURE(ID_AA64PFR0_EL2));
> >>> +		/* 64bit EL2/EL3 only */
> >>> +		val |= FIELD_PREP(FEATURE(ID_AA64PFR0_EL2), 0b0001);
> >>> +		val |= FIELD_PREP(FEATURE(ID_AA64PFR0_EL3), 0b0001);
> >>> +		break;
> >>> +
> >>> +	case SYS_ID_AA64PFR1_EL1:
> >>> +		/* Only support SSBS */
> >>> +		val &= FEATURE(ID_AA64PFR1_SSBS);
> >>> +		break;
> >>> +
> >>> +	case SYS_ID_AA64MMFR0_EL1:
> >>> +		/* Hide ECV, FGT, ExS, Secure Memory */
> >>> +		val &= ~(GENMASK_ULL(63, 43)			|
> >>> +			 FEATURE(ID_AA64MMFR0_TGRAN4_2)		|
> >>> +			 FEATURE(ID_AA64MMFR0_TGRAN16_2)	|
> >>> +			 FEATURE(ID_AA64MMFR0_TGRAN64_2)	|
> >>> +			 FEATURE(ID_AA64MMFR0_SNSMEM));
> >>> +
> >>> +		/* Disallow unsupported S2 page sizes */
> >>> +		switch (PAGE_SIZE) {
> >>> +		case SZ_64K:
> >>> +			val |= FIELD_PREP(FEATURE(ID_AA64MMFR0_TGRAN16_2), 0b0001);
> >>> +			fallthrough;
> >>> +		case SZ_16K:
> >>> +			val |= FIELD_PREP(FEATURE(ID_AA64MMFR0_TGRAN4_2), 0b0001);
> >>> +			fallthrough;
> >>> +		case SZ_4K:
> >>> +			/* Support everything */
> >>> +			break;
> >>> +		}
> >> 
> >> It seems to me that Host hypervisor(L0) has to boot with 4KB page size
> >> to support all (4, 16 and 64KB) page sizes at L1, any specific reason
> >> for this restriction?
> > 
> > Well, yes.
> > 
> > If you have a L0 that has booted with (let's say) 64kB page size, how
> > do you provide S2 mappings with 4kB granularity so that you can
> > implement the permissions that a L1 guest hypervisor can impose on its
> > own guest, given that KVM currently mandates S1 and S2 to use the same
> > page sizes?
> > 
> > You can't. That's why we tell the guest hypervisor how much we
> > support, and the guest hypervisor can decide to go ahead or not
> > depending on what it does.
> > 
> > If one day we can support S2 mappings that are smaller than the host
> > page sizes, then we'll be able to allow to advertise all page sizes.
> > But I wouldn't hold my breath for this to happen.
> 
> Thanks for the detailed explanation!.
> Can we put one line comment that explains why this manipulation?
> It would be helpful to see a comment like S2 PAGE_SIZE should be
> at-least the size of Host PAGE_SIZE?

Can do, but we need to get the terminology straight, because this is
very quickly becoming confusing. Something like:

diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 7c9dd1edf011..d35a947f5679 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -850,7 +850,12 @@ void access_nested_id_reg(struct kvm_vcpu *v, struct sys_reg_params *p,
 			/* Support everything */
 			break;
 		}
-		/* Advertize supported S2 page sizes */
+		/*
+		 * Since we can't support a guest S2 page size smaller than
+		 * the host's own page size (due to KVM only populating its
+		 * own S2 using the kernel's page size), advertise the
+		 * limitation using FEAT_GTG.
+		 */
 		switch (PAGE_SIZE) {
 		case SZ_4K:
 			val |= FIELD_PREP(FEATURE(ID_AA64MMFR0_TGRAN4_2), 0b0010);

So not quite a one line comment! ;-)

Ultimately, all there is to know is in the description of FEAT_GTG in
the ARMv8 ARM.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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