On Fri, Jan 18, 2019 at 05:58:33PM +0000, Marc Zyngier wrote:
> On 17/01/2019 20:33, 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 divided up into slices as noted above: the i
> > parameter denotes the slice index.
> >
> > For simplicity, bits or slices that exceed the maximum vector
> > length supported for the vcpu are ignored for KVM_SET_ONE_REG, and
> > read as zero for KVM_GET_ONE_REG.
> >
> > For the current architecture, only slice i = 0 is significant. The
> > interface design allows i to increase to up to 31 in the future if
> > required by future architectural amendments.
> >
> > 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. In all cases, surplus slices are not enumerated by
> > KVM_GET_REG_LIST.
> >
> > 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 emluation
>
> nit: emulation.
>
> > in the kernel.
> >
> > Signed-off-by: Dave Martin <dave.mar...@arm.com>
> > ---
> > arch/arm64/include/uapi/asm/kvm.h | 10 +++
> > arch/arm64/kvm/guest.c | 131
> > ++++++++++++++++++++++++++++++++++----
> > 2 files changed, 129 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h
> > b/arch/arm64/include/uapi/asm/kvm.h
> > index 97c3478..1ff68fa 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -226,6 +226,16 @@ struct kvm_vcpu_events {
> > KVM_REG_ARM_FW | ((r) & 0xffff))
> > #define KVM_REG_ARM_PSCI_VERSION KVM_REG_ARM_FW_REG(0)
> >
> > +/* SVE registers */
> > +#define KVM_REG_ARM64_SVE (0x15 << KVM_REG_ARM_COPROC_SHIFT)
> > +#define KVM_REG_ARM64_SVE_ZREG(n, i) (KVM_REG_ARM64 |
> > KVM_REG_ARM64_SVE | \
> > + KVM_REG_SIZE_U2048 | \
> > + ((n) << 5) | (i))
> > +#define KVM_REG_ARM64_SVE_PREG(n, i) (KVM_REG_ARM64 |
> > KVM_REG_ARM64_SVE | \
> > + KVM_REG_SIZE_U256 | \
> > + ((n) << 5) | (i) | 0x400)
>
> Can we please have a name for this 0x400 bit?
>
>
> > +#define KVM_REG_ARM64_SVE_FFR(i) KVM_REG_ARM64_SVE_PREG(16, i)
> > +
> > /* Device Control API: ARM VGIC */
> > #define KVM_DEV_ARM_VGIC_GRP_ADDR 0
> > #define KVM_DEV_ARM_VGIC_GRP_DIST_REGS 1
> > diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> > index ffa38d4..b8f9c1e 100644
> > --- a/arch/arm64/kvm/guest.c
> > +++ b/arch/arm64/kvm/guest.c
> > @@ -19,8 +19,10 @@
> > * along with this program. If not, see <http://www.gnu.org/licenses/>.
> > */
> >
> > +#include <linux/bits.h>
> > #include <linux/errno.h>
> > #include <linux/err.h>
> > +#include <linux/kernel.h>
> > #include <linux/kvm_host.h>
> > #include <linux/module.h>
> > #include <linux/vmalloc.h>
> > @@ -28,9 +30,12 @@
> > #include <kvm/arm_psci.h>
> > #include <asm/cputype.h>
> > #include <linux/uaccess.h>
> > +#include <asm/fpsimd.h>
> > #include <asm/kvm.h>
> > #include <asm/kvm_emulate.h>
> > #include <asm/kvm_coproc.h>
> > +#include <asm/kvm_host.h>
> > +#include <asm/sigcontext.h>
> >
> > #include "trace.h"
> >
> > @@ -210,6 +215,108 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const
> > struct kvm_one_reg *reg)
> > return err;
> > }
> >
> > +struct kreg_region {
> > + char *kptr;
>
> Should this rather be void * instead?
This was to support address arithmetic on kptr without explicit casting.
Maybe this is ill-advised. GCC supports pointer arithmetic on void *
with equivalent behaviour as if the pointer were to a character type,
without any cast.
If we rely elsewhere on this working, then we can do it here.
However... [see below]
> > + size_t size;
> > + size_t zeropad;
> > +};
> > +
> > +#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)
> > +
> > +static int sve_reg_region(struct kreg_region *b,
> > + const struct kvm_vcpu *vcpu,
> > + const struct kvm_one_reg *reg)
> > +{
> > + const unsigned int vl = vcpu->arch.sve_max_vl;
> > + const unsigned int vq = sve_vq_from_vl(vl);
> > +
> > + const unsigned int reg_num =
> > + (reg->id & SVE_REG_ID_MASK) >> SVE_REG_ID_SHIFT;
> > + const unsigned int slice_num =
> > + (reg->id & SVE_REG_SLICE_MASK) >> SVE_REG_SLICE_SHIFT;
> > +
> > + unsigned int slice_size, offset, limit;
> > +
> > + if (reg->id >= KVM_REG_ARM64_SVE_ZREG(0, 0) &&
> > + reg->id <= KVM_REG_ARM64_SVE_ZREG(SVE_NUM_ZREGS - 1,
> > + SVE_NUM_SLICES - 1)) {
> > + slice_size = KVM_REG_SIZE(KVM_REG_ARM64_SVE_ZREG(0, 0));
> > +
> > + /* Compute start and end of the register: */
> > + offset = SVE_SIG_ZREG_OFFSET(vq, reg_num) - SVE_SIG_REGS_OFFSET;
> > + limit = offset + SVE_SIG_ZREG_SIZE(vq);
> > +
> > + offset += slice_size * slice_num; /* start of requested slice */
> > +
> > + } else if (reg->id >= KVM_REG_ARM64_SVE_PREG(0, 0) &&
> > + reg->id <= KVM_REG_ARM64_SVE_FFR(SVE_NUM_SLICES - 1)) {
> > + /* (FFR is P16 for our purposes) */
> > +
> > + slice_size = KVM_REG_SIZE(KVM_REG_ARM64_SVE_PREG(0, 0));
> > +
> > + /* Compute start and end of the register: */
> > + offset = SVE_SIG_PREG_OFFSET(vq, reg_num) - SVE_SIG_REGS_OFFSET;
> > + limit = offset + SVE_SIG_PREG_SIZE(vq);
> > +
> > + offset += slice_size * slice_num; /* start of requested slice */
> > +
> > + } else {
> > + return -ENOENT;
> > + }
> > +
> > + b->kptr = (char *)vcpu->arch.sve_state + offset;
>
> I'm very uneasy with this pointer that, at this stage, may point to an
> arbitrary location in memory, given that the slice number is controlled
> by userspace. It feels like a disaster waiting to happen as a potential
> spectre-v1 gadget.
Hmmm, yes, that does seem a concern. We only guard against accessing
that address with an if(), so while the compiler cannot emit an explicit
access, the CPU can emit a speculative read.
So, a speculation-safe bounds check would be appropriate here, at
minimum.
> And actually, we don't have any requirement to handle non-zero slices,
> do we? The important thing is that you've nicely reserved extra space in
> the encoding, and that we can expand the API in a very straightforward way.
>
> So at this stage, I'd drop all support for non-zero slices and return an
> error instead.
Since the extra-slices thing is for the far future (or never), we can
probably drop this code for now.
This means that we can drop the enumeration of extra slices too.
If the vector length is > 256 bytes, I'll do pr_warn() and clamp the
vector lengths seen by the guest to <= 256 so that no extra slices are
needed.
In any case, we can keep the space reserved in the register ID encoding
to allow larger vectors to be supported later if need be.
I'll write up the rule for determining the number of slices somewhere,
but in userspace it will never be effectively tested. Too bad, I
guess: we might add an explicit capability for larger vectors if we
eventually risk hitting incompatibilities here.
Cheers
---Dave
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