Subject: Re: [PATCH v5 12/69] KVM: arm64: nv: Handle HCR_EL2.NV system register traps

From: Marc Zyngier <[email-masked]>
Date: Wed, 26 Jan 2022 16:08:02 +0000

On Mon, 17 Jan 2022 11:31:41 +0000,
Alexandru Elisei <[email-masked]> wrote:
> 
> Hi Marc,
> 
> Nitpick, but HCR_EL2.NV also traps accesses to *_EL02 and *_EL12 registers,
> according to ARM DDI 0487G.a, page D5-2770. The subject could be changed to
> "Handle HCR_EL2.NV *_EL2 system register traps" to better match the
> content, but doesn't make much a difference overall.
> 
> On Mon, Nov 29, 2021 at 08:00:53PM +0000, Marc Zyngier wrote:
> > From: Jintack Lim <[email-masked]>
> > 
> > ARM v8.3 introduces a new bit in the HCR_EL2, which is the NV bit. When
> > this bit is set, accessing EL2 registers in EL1 traps to EL2. In
> > addition, executing the following instructions in EL1 will trap to EL2:
> > tlbi, at, eret, and msr/mrs instructions to access SP_EL1. Most of the
> > instructions that trap to EL2 with the NV bit were undef at EL1 prior to
> > ARM v8.3. The only instruction that was not undef is eret.
> > 
> > This patch sets up a handler for EL2 registers and SP_EL1 register
> > accesses at EL1. The host hypervisor keeps those register values in
> > memory, and will emulate their behavior.
> > 
> > This patch doesn't set the NV bit yet. It will be set in a later patch
> > once nested virtualization support is completed.
> > 
> > Signed-off-by: Jintack Lim <[email-masked]>
> > [maz: added SCTLR_EL2 RES0/RES1 handling]
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > ---
> >  arch/arm64/include/asm/sysreg.h |  41 +++++++++++-
> >  arch/arm64/kvm/sys_regs.c       | 109 ++++++++++++++++++++++++++++++--
> >  2 files changed, 144 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> > index 615dd6278f8b..c77fe5401826 100644
> > --- a/arch/arm64/include/asm/sysreg.h
> > +++ b/arch/arm64/include/asm/sysreg.h
> > @@ -531,10 +531,26 @@
> >  
> >  #define SYS_PMCCFILTR_EL0		sys_reg(3, 3, 14, 15, 7)
> >  
> > +#define SYS_VPIDR_EL2			sys_reg(3, 4, 0, 0, 0)
> > +#define SYS_VMPIDR_EL2			sys_reg(3, 4, 0, 0, 5)
> > +
> >  #define SYS_SCTLR_EL2			sys_reg(3, 4, 1, 0, 0)
> > +#define SYS_ACTLR_EL2			sys_reg(3, 4, 1, 0, 1)
> > +#define SYS_HCR_EL2			sys_reg(3, 4, 1, 1, 0)
> > +#define SYS_MDCR_EL2			sys_reg(3, 4, 1, 1, 1)
> > +#define SYS_CPTR_EL2			sys_reg(3, 4, 1, 1, 2)
> > +#define SYS_HSTR_EL2			sys_reg(3, 4, 1, 1, 3)
> >  #define SYS_HFGRTR_EL2			sys_reg(3, 4, 1, 1, 4)
> >  #define SYS_HFGWTR_EL2			sys_reg(3, 4, 1, 1, 5)
> >  #define SYS_HFGITR_EL2			sys_reg(3, 4, 1, 1, 6)
> > +#define SYS_HACR_EL2			sys_reg(3, 4, 1, 1, 7)
> > +
> > +#define SYS_TTBR0_EL2			sys_reg(3, 4, 2, 0, 0)
> > +#define SYS_TTBR1_EL2			sys_reg(3, 4, 2, 0, 1)
> > +#define SYS_TCR_EL2			sys_reg(3, 4, 2, 0, 2)
> > +#define SYS_VTTBR_EL2			sys_reg(3, 4, 2, 1, 0)
> > +#define SYS_VTCR_EL2			sys_reg(3, 4, 2, 1, 2)
> > +
> >  #define SYS_ZCR_EL2			sys_reg(3, 4, 1, 2, 0)
> >  #define SYS_TRFCR_EL2			sys_reg(3, 4, 1, 2, 1)
> >  #define SYS_DACR32_EL2			sys_reg(3, 4, 3, 0, 0)
> > @@ -543,14 +559,26 @@
> >  #define SYS_HAFGRTR_EL2			sys_reg(3, 4, 3, 1, 6)
> >  #define SYS_SPSR_EL2			sys_reg(3, 4, 4, 0, 0)
> >  #define SYS_ELR_EL2			sys_reg(3, 4, 4, 0, 1)
> > +#define SYS_SP_EL1			sys_reg(3, 4, 4, 1, 0)
> >  #define SYS_IFSR32_EL2			sys_reg(3, 4, 5, 0, 1)
> > +#define SYS_AFSR0_EL2			sys_reg(3, 4, 5, 1, 0)
> > +#define SYS_AFSR1_EL2			sys_reg(3, 4, 5, 1, 1)
> >  #define SYS_ESR_EL2			sys_reg(3, 4, 5, 2, 0)
> >  #define SYS_VSESR_EL2			sys_reg(3, 4, 5, 2, 3)
> >  #define SYS_FPEXC32_EL2			sys_reg(3, 4, 5, 3, 0)
> >  #define SYS_TFSR_EL2			sys_reg(3, 4, 5, 6, 0)
> >  #define SYS_FAR_EL2			sys_reg(3, 4, 6, 0, 0)
> >  
> > -#define SYS_VDISR_EL2			sys_reg(3, 4, 12, 1,  1)
> > +#define SYS_FAR_EL2			sys_reg(3, 4, 6, 0, 0)
> > +#define SYS_HPFAR_EL2			sys_reg(3, 4, 6, 0, 4)
> > +
> > +#define SYS_MAIR_EL2			sys_reg(3, 4, 10, 2, 0)
> > +#define SYS_AMAIR_EL2			sys_reg(3, 4, 10, 3, 0)
> > +
> > +#define SYS_VBAR_EL2			sys_reg(3, 4, 12, 0, 0)
> > +#define SYS_RVBAR_EL2			sys_reg(3, 4, 12, 0, 1)
> > +#define SYS_RMR_EL2			sys_reg(3, 4, 12, 0, 2)
> > +#define SYS_VDISR_EL2			sys_reg(3, 4, 12, 1, 1)
> >  #define __SYS__AP0Rx_EL2(x)		sys_reg(3, 4, 12, 8, x)
> >  #define SYS_ICH_AP0R0_EL2		__SYS__AP0Rx_EL2(0)
> >  #define SYS_ICH_AP0R1_EL2		__SYS__AP0Rx_EL2(1)
> > @@ -592,15 +620,24 @@
> >  #define SYS_ICH_LR14_EL2		__SYS__LR8_EL2(6)
> >  #define SYS_ICH_LR15_EL2		__SYS__LR8_EL2(7)
> >  
> > +#define SYS_CONTEXTIDR_EL2		sys_reg(3, 4, 13, 0, 1)
> > +#define SYS_TPIDR_EL2			sys_reg(3, 4, 13, 0, 2)
> > +
> > +#define SYS_CNTVOFF_EL2			sys_reg(3, 4, 14, 0, 3)
> > +#define SYS_CNTHCTL_EL2			sys_reg(3, 4, 14, 1, 0)
> > +
> >  /* VHE encodings for architectural EL0/1 system registers */
> >  #define SYS_SCTLR_EL12			sys_reg(3, 5, 1, 0, 0)
> >  #define SYS_CPACR_EL12			sys_reg(3, 5, 1, 0, 2)
> >  #define SYS_ZCR_EL12			sys_reg(3, 5, 1, 2, 0)
> > +
> >  #define SYS_TTBR0_EL12			sys_reg(3, 5, 2, 0, 0)
> >  #define SYS_TTBR1_EL12			sys_reg(3, 5, 2, 0, 1)
> >  #define SYS_TCR_EL12			sys_reg(3, 5, 2, 0, 2)
> > +
> >  #define SYS_SPSR_EL12			sys_reg(3, 5, 4, 0, 0)
> >  #define SYS_ELR_EL12			sys_reg(3, 5, 4, 0, 1)
> > +
> >  #define SYS_AFSR0_EL12			sys_reg(3, 5, 5, 1, 0)
> >  #define SYS_AFSR1_EL12			sys_reg(3, 5, 5, 1, 1)
> >  #define SYS_ESR_EL12			sys_reg(3, 5, 5, 2, 0)
> > @@ -618,6 +655,8 @@
> >  #define SYS_CNTV_CTL_EL02		sys_reg(3, 5, 14, 3, 1)
> >  #define SYS_CNTV_CVAL_EL02		sys_reg(3, 5, 14, 3, 2)
> >  
> > +#define SYS_SP_EL2			sys_reg(3, 6,  4, 1, 0)
> 
> Checked the encoding for the newly added registers, they match.
> 
> > +
> >  /* Common SCTLR_ELx flags. */
> >  #define SCTLR_ELx_DSSBS	(BIT(44))
> >  #define SCTLR_ELx_ATA	(BIT(43))
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index e3ec1a44f94d..a23701f29858 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -105,6 +105,46 @@ static u32 get_ccsidr(u32 csselr)
> >  	return ccsidr;
> >  }
> >  
> > +static bool access_rw(struct kvm_vcpu *vcpu,
> > +		      struct sys_reg_params *p,
> > +		      const struct sys_reg_desc *r)
> > +{
> > +	if (p->is_write)
> > +		vcpu_write_sys_reg(vcpu, p->regval, r->reg);
> > +	else
> > +		p->regval = vcpu_read_sys_reg(vcpu, r->reg);
> > +
> > +	return true;
> > +}
> > +
> > +static bool access_sctlr_el2(struct kvm_vcpu *vcpu,
> > +			     struct sys_reg_params *p,
> > +			     const struct sys_reg_desc *r)
> > +{
> > +	if (p->is_write) {
> > +		u64 val = p->regval;
> > +
> > +		if (vcpu_el2_e2h_is_set(vcpu) && vcpu_el2_tge_is_set(vcpu)) {
> > +			val &= ~(GENMASK_ULL(63,45) | GENMASK_ULL(34, 32) |
> > +				 BIT_ULL(17) | BIT_ULL(9));
> > +			val |=  SCTLR_EL1_RES1;
> > +		} else {
> > +			val &= ~(GENMASK_ULL(63,45) | BIT_ULL(42) |
> > +				 GENMASK_ULL(39, 38) | GENMASK_ULL(35, 32) |
> > +				 BIT_ULL(26) | BIT_ULL(24) | BIT_ULL(20) |
> > +				 BIT_ULL(17) | GENMASK_ULL(15, 14) |
> > +				 GENMASK(10, 7));
> > +			val |=  SCTLR_EL2_RES1;
> > +		}
> 
> Some bits in SCTLR_EL2 are functional bits when {E2H, TGE} = {1, 1}, otherwise
> they are RES0. This is how ARM DDI 0487G.a describes the behaviour of bits which
> are RES0 only in some contexts (page Glossary-8529, emphasis added by me):
> 
> "For a bit in a read/write register, when the bit is described as RES0:
> 
> - An indirect write to the register sets the bit to 0.
> 
> - ** A read of the bit must return the value last successfully written to the bit,
>   by either a direct or an indirect write, regardless of the use of the register
>   when the bit was written **
> 
> If the bit has not been successfully written since reset, then the read of the bit returns the reset
> value if there is one, or otherwise returns an UNKNOWN value.
> 
> - A direct write to the bit must update a storage location associated with the bit.
> 
> - While the use of the register is such that the bit is described as RES0, the
>   value of the bit must have no effect on the operation of the PE, other than
>   determining the value read back from that bit, unless this Manual
>   explicitly defines additional properties for the bit."
> 
> Let's take bit 24, E0E, as an example. When E2H,TGE != {1,1} (which means the
> bit is now RES0), KVM clears the bit on a write, when according to the above
> definition, it should save the value.

That's an interesting observation. It means we should munge these bits
at save/restore time (specially in the nVHE case where we have to
translate SCTLR_EL2 to the SCTLR_EL1 format), rather than at access
time.

This would also solve the discrepancy we have between the ARMv8.3
trapping and the ARMv8.4 memory write (we can sanitise the former, but
not the latter).

Thanks,

	M.

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

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel