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

[Sebastian Ott <sebott@redhat.com>]

Hi,

On Wed, 14 Aug 2024, Shameerali Kolothum Thodi wrote:
>>
>> 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.
>
> Thanks Marc for explaining this. I was not aware of this one.
>
>> I really don't see how you can safely do that without completely
>> changing the way we handle the debug registers.
>
> Looks like Reji has attempted to do this a while back,
> https://lore.kernel.org/kvm/20220419065544.3616948-13-reijiw@google.com/
>

I've got two machines that differ in the number of breakpoints and
it would be nice to be able to migrate between these. Is anything
preventing us from trapping the access and make sure the correct
breakpoint is used? Is anyone working on this? If not I'd like to
give it a shot.

Thanks,
Sebastian