[PATCH v2] KVM: arm64: Make the exposed feature bits in AA64DFR0_EL1 writable from userspace
Oliver Upton
oliver.upton at linux.dev
Thu Aug 15 10:42:22 PDT 2024
On Thu, Aug 15, 2024 at 04:59:54PM +0100, Shameer Kolothum 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 differ. Add support to make this feature
> writable from userspace by setting the mask bit. While at it, set the
> mask bits for the exposed WRPs(Number of Watchpoints) as well.
> Also update the selftest to cover these fields.
>
> However we still can't make BRPs and CTX_CMPs fields writable, because
> as per ARM ARM DDI 0487K.a, section G2.8.2 Breakpoint types and
> linking of breakpoints, highest numbered breakpoints must be context aware
> breakpoints. And it will be problematic if userspace decreases the number
> of non-context aware breakpoints as it will make the context aware
> breakpoints for the guest mapped to a wrong one.
>
> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi at huawei.com>
> ---
> v1 --> v2:
> Removed making BRPs and CTX_CMPs writable.
> v1: https://lore.kernel.org/all/20240813142835.77180-1-shameerali.kolothum.thodi@huawei.com/
> ---
> arch/arm64/kvm/sys_regs.c | 13 ++++++++++++-
> tools/testing/selftests/kvm/aarch64/set_id_regs.c | 2 ++
> 2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c90324060436..e77cd6d1abb5 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2376,7 +2376,18 @@ 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 |
> + /*
> + * We can't still make BRPs and CTX_CMPx writable as highest
> + * numbered breakpoints must be context aware breakpoints(ARM ARM
> + * DDI 0487K.a, section G2.8.2 Breakpoint types and linking of
> + * breakpoints). Hence, if the number of non-context aware breakpoints
> + * for the Guest is decreased by userspace, that will be problematic
> + * as KVM will map context aware breakpoints for the vCPU to different
> + * numbered breakpoints for the pCPU.
A few minor nitpicks:
- BRPs counts the total number of breakpoints, not just the non-context
aware BRPs
- KVM doesn't do any register mapping at all, which is why we disallow
changing the breakpoints altogether
- The citation is for the AArch32 view of the debug architecture, not
AArch64.
Perhaps use this wording:
/*
* Prior to FEAT_Debugv8.9, the architecture defines context-aware
* breakpoints (CTX_CMPs) as the highest numbered breakpoints (BRPs).
* KVM does not trap + emulate the breakpoint registers, and as such
* cannot support a layout that misaligns with the underlying hardware.
* While it may be possible to describe a subset that aligns with
* hardware, just prevent changes to BRPs and CTX_CMPs altogether for
* simplicity.
*
* See DDI0487K.a, section D2.8.3 Breakpoint types and linking
* of breakpoints for more details.
*/
Besides that, LGTM:
Reviewed-by: Oliver Upton <oliver.upton at linux.dev>
> + */
> + .val = ID_AA64DFR0_EL1_DoubleLock_MASK |
> + ID_AA64DFR0_EL1_WRPs_MASK |
> + ID_AA64DFR0_EL1_PMUVer_MASK |
> ID_AA64DFR0_EL1_DebugVer_MASK, },
> ID_SANITISED(ID_AA64DFR1_EL1),
> ID_UNALLOCATED(5,2),
> diff --git a/tools/testing/selftests/kvm/aarch64/set_id_regs.c b/tools/testing/selftests/kvm/aarch64/set_id_regs.c
> index d20981663831..6edc5412abe8 100644
> --- a/tools/testing/selftests/kvm/aarch64/set_id_regs.c
> +++ b/tools/testing/selftests/kvm/aarch64/set_id_regs.c
> @@ -68,6 +68,8 @@ struct test_feature_reg {
> }
>
> static const struct reg_ftr_bits ftr_id_aa64dfr0_el1[] = {
> + S_REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64DFR0_EL1, DoubleLock, 0),
> + REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64DFR0_EL1, WRPs, 0),
> S_REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64DFR0_EL1, PMUVer, 0),
> REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64DFR0_EL1, DebugVer, ID_AA64DFR0_EL1_DebugVer_IMP),
> REG_FTR_END,
> --
> 2.45.2
>
--
Thanks,
Oliver
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