Re: [PATCH v1 2/4] KVM: arm64: Document KVM_ARM_GET_REG_WRITABLE_MASKS

[Marc Zyngier <maz@kernel.org>]

Hey Eric,

On Tue, 13 Feb 2024 13:59:31 +0000,
Eric Auger <eauger@redhat.com> wrote:
> 
> Hi,
> 
> On 9/19/23 19:50, Jing Zhang wrote:
> > Add some basic documentation on how to get feature ID register writable
> > masks from userspace.
> > 
> > Signed-off-by: Jing Zhang <jingzhangos@google.com>
> > ---
> >  Documentation/virt/kvm/api.rst | 42 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 42 insertions(+)
> > 
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 21a7578142a1..2defb5e198ce 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6070,6 +6070,48 @@ writes to the CNTVCT_EL0 and CNTPCT_EL0 registers using the SET_ONE_REG
> >  interface. No error will be returned, but the resulting offset will not be
> >  applied.
> >  
> > +4.139 KVM_ARM_GET_REG_WRITABLE_MASKS
> > +-------------------------------------------
> > +
> > +:Capability: KVM_CAP_ARM_SUPPORTED_FEATURE_ID_RANGES
> > +:Architectures: arm64
> > +:Type: vm ioctl
> > +:Parameters: struct reg_mask_range (in/out)
> > +:Returns: 0 on success, < 0 on error
> > +
> > +
> > +::
> > +
> > +        #define ARM64_FEATURE_ID_SPACE_SIZE	(3 * 8 * 8)
> > +        #define ARM64_FEATURE_ID_RANGE_IDREGS	BIT(0)
> > +
> > +        struct reg_mask_range {
> > +                __u64 addr;             /* Pointer to mask array */
> > +                __u32 range;            /* Requested range */
> > +                __u32 reserved[13];
> > +        };
> > +
> > +This ioctl copies the writable masks for Feature ID registers to userspace.
> > +The Feature ID space is defined as the AArch64 System register space with
> > +op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7}, op2=={0-7}.
> when attempting a migration between Ampere Altra and ThunderXv2 the
> first hurdle is to handle a failure when writing ICC_CTLR_EL1
> (3.0.12.12.4) on dest. This reg is outside of the scope of the above
> single range (BIT(0)).

Indeed. But more importantly, this isn't really an ID register. Plenty
of variable bits in there.

> This may be questionable if we want to migrate between those types of
> machines but the goal is to exercise different scenarios to have a
> gloval view of the problems.

I think this is a valuable experiment, and we should definitely
explore this sort of things (as I cannot see the diversity of ARM
system slowing down any time soon).

> 
> This reg exposes some RO capabilities such as ExtRange, A3V, SEIS,
> IDBits, ...
> So to get the migration going further I would need to tweek this on the
> source - for instance I guess SEIS could be reset despite the host HW
> cap - without making too much trouble.

I'm not sure SEIS is such an easy one: if you promised the guest that
it would never get an SError doing the most stupid things (SEIS=0), it
really shouldn't get one after migration. If you advertised it on the
source HW (Altra), a migration to TX2 would be fine.

The other bits are possible to change depending on the requirements of
the VM (aff3, IDBits), and ExtRange should always be set to 0 (because
our GIC implementation doesn't support EPPI/ESPI).

The really ugly part here is that if you want to affect these bits,
you will need to trap and emulate the access. Not a big deal, but in
the absence of FGT, you will need to handle the full Common trap
group, which is going to slow things down (you will have to trap
ICV_PMR_EL1, for example).

> What would you recommend, adding a new range? But I guess we need to
> design ranges carefully otherwise we may be quickly limited by the
> number of flag bits.

I can see a need to adding a range that would cover non-ID registers
that have RO fields. But we also need to consider the case of EL2
registers that take part in this.

For example, ICV_CTLR_EL1 and ICH_VTR_EL2 and deeply linked, and share
some fields. Without NV, no need to expose HCR_VTR_EL2. With NV, this
register actually drives ICV_CTLR_EL1.

So careful planning is required here, and the impact of this measured.

Thanks,

	M.

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

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