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

From: "Russell King (Oracle)" <[email-masked]>
Date: Tue, 18 Jan 2022 15:51:30 +0000

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]>
> ---
...
> @@ -1825,9 +1882,51 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	{ PMU_SYS_REG(SYS_PMCCFILTR_EL0), .access = access_pmu_evtyper,
>  	  .reset = reset_val, .reg = PMCCFILTR_EL0, .val = 0 },
>  
> +	{ SYS_DESC(SYS_VPIDR_EL2), access_rw, reset_val, VPIDR_EL2, 0 },
> +	{ SYS_DESC(SYS_VMPIDR_EL2), access_rw, reset_val, VMPIDR_EL2, 0 },
> +
> +	{ SYS_DESC(SYS_SCTLR_EL2), access_sctlr_el2, reset_val, SCTLR_EL2, SCTLR_EL2_RES1 },
> +	{ SYS_DESC(SYS_ACTLR_EL2), access_rw, reset_val, ACTLR_EL2, 0 },
> +	{ SYS_DESC(SYS_HCR_EL2), access_rw, reset_val, HCR_EL2, 0 },
> +	{ SYS_DESC(SYS_MDCR_EL2), access_rw, reset_val, MDCR_EL2, 0 },
> +	{ SYS_DESC(SYS_CPTR_EL2), access_rw, reset_val, CPTR_EL2, CPTR_EL2_DEFAULT },
> +	{ SYS_DESC(SYS_HSTR_EL2), access_rw, reset_val, HSTR_EL2, 0 },
> +	{ SYS_DESC(SYS_HACR_EL2), access_rw, reset_val, HACR_EL2, 0 },
> +
> +	{ SYS_DESC(SYS_TTBR0_EL2), access_rw, reset_val, TTBR0_EL2, 0 },
> +	{ SYS_DESC(SYS_TTBR1_EL2), access_rw, reset_val, TTBR1_EL2, 0 },
> +	{ SYS_DESC(SYS_TCR_EL2), access_rw, reset_val, TCR_EL2, TCR_EL2_RES1 },
> +	{ SYS_DESC(SYS_VTTBR_EL2), access_rw, reset_val, VTTBR_EL2, 0 },
> +	{ SYS_DESC(SYS_VTCR_EL2), access_rw, reset_val, VTCR_EL2, 0 },
> +
>  	{ SYS_DESC(SYS_DACR32_EL2), NULL, reset_unknown, DACR32_EL2 },
> +	{ SYS_DESC(SYS_SPSR_EL2), access_rw, reset_val, SPSR_EL2, 0 },
> +	{ SYS_DESC(SYS_ELR_EL2), access_rw, reset_val, ELR_EL2, 0 },
> +	{ SYS_DESC(SYS_SP_EL1), access_sp_el1},
> +
>  	{ SYS_DESC(SYS_IFSR32_EL2), NULL, reset_unknown, IFSR32_EL2 },
> +	{ SYS_DESC(SYS_AFSR0_EL2), access_rw, reset_val, AFSR0_EL2, 0 },
> +	{ SYS_DESC(SYS_AFSR1_EL2), access_rw, reset_val, AFSR1_EL2, 0 },
> +	{ SYS_DESC(SYS_ESR_EL2), access_rw, reset_val, ESR_EL2, 0 },
>  	{ SYS_DESC(SYS_FPEXC32_EL2), NULL, reset_val, FPEXC32_EL2, 0x700 },
> +
> +	{ SYS_DESC(SYS_FAR_EL2), access_rw, reset_val, FAR_EL2, 0 },
> +	{ SYS_DESC(SYS_HPFAR_EL2), access_rw, reset_val, HPFAR_EL2, 0 },
> +
> +	{ SYS_DESC(SYS_MAIR_EL2), access_rw, reset_val, MAIR_EL2, 0 },
> +	{ SYS_DESC(SYS_AMAIR_EL2), access_rw, reset_val, AMAIR_EL2, 0 },
> +
> +	{ SYS_DESC(SYS_VBAR_EL2), access_rw, reset_val, VBAR_EL2, 0 },
> +	{ SYS_DESC(SYS_RVBAR_EL2), access_rw, reset_val, RVBAR_EL2, 0 },
> +	{ SYS_DESC(SYS_RMR_EL2), trap_undef },
> +
> +	{ SYS_DESC(SYS_CONTEXTIDR_EL2), access_rw, reset_val, CONTEXTIDR_EL2, 0 },
> +	{ SYS_DESC(SYS_TPIDR_EL2), access_rw, reset_val, TPIDR_EL2, 0 },
> +
> +	{ SYS_DESC(SYS_CNTVOFF_EL2), access_rw, reset_val, CNTVOFF_EL2, 0 },
> +	{ SYS_DESC(SYS_CNTHCTL_EL2), access_rw, reset_val, CNTHCTL_EL2, 0 },
> +
> +	{ SYS_DESC(SYS_SP_EL2), NULL, reset_unknown, SP_EL2 },

Doesn't this have an effect on the ability to migrate guests between
identical hardware but running kernels with vs without this patch?
>From what I remember, QEMU fails a migration if the migration target
has less system registers than the migration source.

If so, this should at the very least be spelt out in the commit
message - it's a user experience breaking change. Maybe also preventing
the exposure of these when NV is disabled would be a good idea?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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