Re: [PATCH] KVM: arm64: Make the exposed feature bits in AA64DFR0_EL1 writable from userspace

[Marc Zyngier <maz@kernel.org>]

On Tue, 13 Aug 2024 15:28:35 +0100,
Shameer Kolothum <shameerali.kolothum.thodi@huawei.com> wrote:
> 
> KVM exposes the OS double lock feature bit to Guests but returns
> RAZ/WI on Guest OSDLR_EL1 access. This breaks Guest migration between
> systems where this feature support differ. Add support to make this
> feature writable from userspace by setting the mask bit. While at it,
> set the mask bits for other exposed features in the AA64DFR0_EL1
> register as well.
> 
> Also update the selftest to cover these fields.
> 
> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> ---
>    This is based on the discussion here(Thanks to Oliver),
>    https://lore.kernel.org/all/ZrVSlbVwnaMDShah@linux.dev/
> ---
>  arch/arm64/kvm/sys_regs.c                         | 6 +++++-
>  tools/testing/selftests/kvm/aarch64/set_id_regs.c | 4 ++++
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c90324060436..adb49d681052 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2376,7 +2376,11 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	  .get_user = get_id_reg,
>  	  .set_user = set_id_aa64dfr0_el1,
>  	  .reset = read_sanitised_id_aa64dfr0_el1,
> -	  .val = ID_AA64DFR0_EL1_PMUVer_MASK |
> +	  .val = ID_AA64DFR0_EL1_DoubleLock_MASK |
> +		 ID_AA64DFR0_EL1_CTX_CMPs_MASK |
> +		 ID_AA64DFR0_EL1_WRPs_MASK |
> +		 ID_AA64DFR0_EL1_BRPs_MASK |


I think this is going to cause some troubles.

The issue is that context-aware breakpoints are the highest-numbered
breakpoints, right after the normal breakpoints (D2.8.3 "Breakpoint
types and linking of breakpoints"). So if you reduce the number of
normal breakpoints, you shift the context-aware ones down, and
everything breaks.

I really don't see how you can safely do that without completely
changing the way we handle the debug registers.

Thanks,

	M.

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