On Thu, Feb 21, 2019 at 03:23:37PM +0000, Julien Thierry wrote:
> Hi Dave,
>
> On 18/02/2019 19:52, Dave Martin wrote:
> > This patch adds the following registers for access via the
> > KVM_{GET,SET}_ONE_REG interface:
> >
> > * KVM_REG_ARM64_SVE_ZREG(n, i) (n = 0..31) (in 2048-bit slices)
> > * KVM_REG_ARM64_SVE_PREG(n, i) (n = 0..15) (in 256-bit slices)
> > * KVM_REG_ARM64_SVE_FFR(i) (in 256-bit slices)
> >
> > In order to adapt gracefully to future architectural extensions,
> > the registers are logically divided up into slices as noted above:
> > the i parameter denotes the slice index.
> >
> > This allows us to reserve space in the ABI for future expansion of
> > these registers. However, as of today the architecture does not
> > permit registers to be larger than a single slice, so no code is
> > needed in the kernel to expose additional slices, for now. The
> > code can be extended later as needed to expose them up to a maximum
> > of 32 slices (as carved out in the architecture itself) if they
> > really exist someday.
> >
> > The registers are only visible for vcpus that have SVE enabled.
> > They are not enumerated by KVM_GET_REG_LIST on vcpus that do not
> > have SVE.
> >
> > Accesses to the FPSIMD registers via KVM_REG_ARM_CORE is not
> > allowed for SVE-enabled vcpus: SVE-aware userspace can use the
> > KVM_REG_ARM64_SVE_ZREG() interface instead to access the same
> > register state. This avoids some complex and pointless emulation
> > in the kernel to convert between the two views of these aliased
> > registers.
> >
> > Signed-off-by: Dave Martin <dave.mar...@arm.com>
> >
> > ---
> >
> > Changes since v4:
> >
> > * Add "BASE" #defines for the Z-reg and P-reg ranges in the KVM
> > register ID space, to make the 0x400 magic number a little less
> > cryptic.
> >
> > * Pull KVM_SVE_{Z,P}REG_SIZE defines from "KVM: arm64: Enumerate SVE
> > register indices for KVM_GET_REG_LIST", since we now use them here.
> >
> > * Simplify sve_reg_region(), and give up on the attempt to make
> > kreg_region a general thing: nothing else will use it for now,
> > anyway, so let's keep it as simple as possible.
> >
> > * Drop support for multiple slices per register. This functionality
> > can be added back in later if needed, without ABI breaks.
> >
> > * Pull vcpu_sve_state_size() into kvm_host.h, from "KVM: arm64/sve:
> > Allow userspace to enable SVE for vcpus". This is needed for use
> > with array_index_nospec() to determine the applicable buffer bounds.
> > To avoid circular header deependency issues, the function is also
> > converted into a macro, but is otherwise equivalent to the original
> > version.
> >
> > * Guard sve_state base offset in kernel memory with
> > array_index_nospec(), since it is generated from user data that can
> > put it out of range.
> >
> > (sve_state will get allocated with the corresponding size later in
> > the series. For now, this code is dormant since no means is
> > provided for userspace to create SVE-enabled vcpus yet.)
> > ---
> > arch/arm64/include/asm/kvm_host.h | 14 ++++
> > arch/arm64/include/uapi/asm/kvm.h | 17 +++++
> > arch/arm64/kvm/guest.c | 138
> > ++++++++++++++++++++++++++++++++++----
> > 3 files changed, 157 insertions(+), 12 deletions(-)
> >
>
> [...]
>
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index f491456..8cfa889 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
>
> [...]
>
> > @@ -211,6 +217,114 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const
> > struct kvm_one_reg *reg)
> > return err;
> > }
> >
> > +#define SVE_REG_SLICE_SHIFT 0
> > +#define SVE_REG_SLICE_BITS 5
> > +#define SVE_REG_ID_SHIFT (SVE_REG_SLICE_SHIFT + SVE_REG_SLICE_BITS)
> > +#define SVE_REG_ID_BITS 5
> > +
> > +#define SVE_REG_SLICE_MASK \
> > + GENMASK(SVE_REG_SLICE_SHIFT + SVE_REG_SLICE_BITS - 1, \
> > + SVE_REG_SLICE_SHIFT)
> > +#define SVE_REG_ID_MASK
> > \
> > + GENMASK(SVE_REG_ID_SHIFT + SVE_REG_ID_BITS - 1, SVE_REG_ID_SHIFT)
> > +
> > +#define SVE_NUM_SLICES (1 << SVE_REG_SLICE_BITS)
> > +
> > +#define KVM_SVE_ZREG_SIZE KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0))
> > +#define KVM_SVE_PREG_SIZE KVM_REG_SIZE(KVM_REG_ARM64_SVE_PREG(0, 0))
> > +
> > +struct sve_state_region {
>
> This sve_state_region feels a bit too generic too me.
>
> So far it is only used to access a single (slice of a) register at a
> time. Is there a plan to use it for more?
It's there as a way to factor out security-sensitive range computations
that we otherwise have to do twice -- I'd rather have the (potential)
bugs in one place. sve_state is particularly awkward because it is
heterogeneous, with variably sized members for which no C declaration
is avaiable (or possible).
Previously it was used in four places, because I tried to emulate the
VFP get/set functions for SVE vcpus. Now that functionality has been
dropped I agree that this function looks like a bit like overkill. But
I didn't come up with a good way to split it without duplicating an
undesirable amount of fiddly logic.
"sve_state" in the name comes from the naming of the kernel field(s)
that this computes ranges on: vcpu->arch.sve_state (and thread->
sve_state, which we don't operate on here, but which has the same
format).
So, this struct describes a slice of "sve_state", hence the name. But
you're right, it is only ever supposed to span a single SVE register
within there.
> Otherwise I'd suggest at least naming it something like sve_reg_region,
> sve_reg_mem_region or sve_reg_mem_desc.
It used to be called struct kreg_region. The name "sve_state_region"
was an attempt to make it look less generic, which doesn't appear to
have worked too well.
Maybe "sve_state_reg_region" would narrow the apparent scope of this a
little further.
Thoughts?
I'll take a look, and at least add a comment explaining what this
struct is supposed to represent.
>
>
> > + unsigned int koffset; /* offset into sve_state in kernel memory */
> > + unsigned int klen; /* length in kernel memory */
> > + unsigned int upad; /* extra trailing padding in user memory */
> > +};
> > +
> > +/* Get sanitised bounds for user/kernel SVE register copy */
> > +static int sve_reg_region(struct sve_state_region *region,
>
> I feel that sve_reg_to_region or sve_reg_get_region would be a clearer name.
..._reg_to_region() seems a good name. I'll pick that up, modulo other
changes.
Cheers
---Dave
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