Re: [PATCH v3 02/28] arm64: KVM: Hide unsupported AArch64 CPU features from guests

Catalin Marinas
2017-10-11T14:14:44Z

On Tue, Oct 10, 2017 at 07:38:19PM +0100, Dave P Martin wrote:
> Currently, a guest kernel sees the true CPU feature registers
> (ID_*_EL1) when it reads them using MRS instructions. This means
> that the guest will observe features that are present in the
> hardware but the host doesn't understand or doesn't provide support
> for. A guest may legimitately try to use such a feature as per the
> architecture, but use of the feature may trap instead of working
> normally, triggering undef injection into the guest.
>
> This is not a problem for the host, but the guest may go wrong when
> running on newer hardware than the host knows about.
>
> This patch hides from guest VMs any AArch64-specific CPU features
> that the host doesn't support, by exposing to the guest the
> sanitised versions of the registers computed by the cpufeatures
> framework, instead of the true hardware registers. To achieve
> this, HCR_EL2.TID3 is now set for AArch64 guests, and emulation
> code is added to KVM to report the sanitised versions of the
> affected registers in response to MRS and register reads from
> userspace.
>
> The affected registers are removed from invariant_sys_regs[] (since
> the invariant_sys_regs handling is no longer quite correct for
> them) and added to sys_reg_desgs[], with appropriate access(),
> get_user() and set_user() methods. No runtime vcpu storage is
> allocated for the registers: instead, they are read on demand from
> the cpufeatures framework. This may need modification in the
> future if there is a need for userspace to customise the features
> visible to the guest.
>
> Attempts by userspace to write the registers are handled similarly
> to the current invariant_sys_regs handling: writes are permitted,
> but only if they don't attempt to change the value. This is
> sufficient to support VM snapshot/restore from userspace.
>
> Because of the additional registers, restoring a VM on an older
> kernel may not work unless userspace knows how to handle the extra
> VM registers exposed to the KVM user ABI by this patch.
>
> Under the principle of least damage, this patch makes no attempt to
> handle any of the other registers currently in
> invariant_sys_regs[], or to emulate registers for AArch32: however,
> these could be handled in a similar way in future, as necessary.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> ---
> arch/arm64/include/asm/sysreg.h | 3 +
> arch/arm64/kvm/hyp/switch.c | 6 +
> arch/arm64/kvm/sys_regs.c | 282 +++++++++++++++++++++++++++++++++-------
> 3 files changed, 246 insertions(+), 45 deletions(-)

Acked-by: Catalin Marinas <catalin.marinas@arm.com>