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SubjectRe: [PATCH v1 15/16] kvm: arm64: Allow configuring physical address space size
On Thu, Feb 08, 2018 at 05:53:17PM +0000, Suzuki K Poulose wrote:
> On 08/02/18 11:14, Christoffer Dall wrote:
> >On Tue, Jan 09, 2018 at 07:04:10PM +0000, Suzuki K Poulose wrote:
> >>Allow the guests to choose a larger physical address space size.
> >>The default and minimum size is 40bits. A guest can change this
> >>right after the VM creation, but before the stage2 entry page
> >>tables are allocated (i.e, before it registers a memory range
> >>or maps a device address). The size is restricted to the maximum
> >>supported by the host. Also, the guest can only increase the PA size,
> >>from the existing value, as reducing it could break the devices which
> >>may have verified their physical address for validity and may do a
> >>lazy mapping(e.g, VGIC).
> >>
> >>Cc: Marc Zyngier <marc.zyngier@arm.com>
> >>Cc: Christoffer Dall <cdall@linaro.org>
> >>Cc: Peter Maydell <peter.maydell@linaro.org>
> >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> >>---
> >> Documentation/virtual/kvm/api.txt | 27 ++++++++++++++++++++++++++
> >> arch/arm/include/asm/kvm_host.h | 7 +++++++
> >> arch/arm64/include/asm/kvm_host.h | 1 +
> >> arch/arm64/include/asm/kvm_mmu.h | 41 ++++++++++++++++++++++++++++++---------
> >> arch/arm64/kvm/reset.c | 28 ++++++++++++++++++++++++++
> >> include/uapi/linux/kvm.h | 4 ++++
> >> virt/kvm/arm/arm.c | 2 +-
> >> 7 files changed, 100 insertions(+), 10 deletions(-)
> >>
> >>diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> >>index 57d3ee9e4bde..a203faf768c4 100644
> >>--- a/Documentation/virtual/kvm/api.txt
> >>+++ b/Documentation/virtual/kvm/api.txt
> >>@@ -3403,6 +3403,33 @@ invalid, if invalid pages are written to (e.g. after the end of memory)
> >> or if no page table is present for the addresses (e.g. when using
> >> hugepages).
> >>+4.109 KVM_ARM_GET_PHYS_SHIFT
> >>+
> >>+Capability: KVM_CAP_ARM_CONFIG_PHYS_SHIFT
> >>+Architectures: arm64
> >>+Type: vm ioctl
> >>+Parameters: __u32 (out)
> >>+Returns: 0 on success, a negative value on error
> >>+
> >>+This ioctl is used to get the current maximum physical address size for
> >>+the VM. The value is Log2(Maximum_Physical_Address). This is neither the
> >>+ amount of physical memory assigned to the VM nor the maximum physical address
> >>+that a real CPU on the host can handle. Rather, this is the upper limit of the
> >>+guest physical address that can be used by the VM.
> >
> >What is the point of this? Presumably if userspace has set the size, it
> >already knows the size?
>
> This can help the userspace know, what the "default" limit is. As such I am
> not particular about keeping this around.
>

Userspace has to already know, since otherwise things don't work today,
so I think we can omit this.

> >
> >>+
> >>+4.109 KVM_ARM_SET_PHYS_SHIFT
> >>+
> >>+Capability: KVM_CAP_ARM_CONFIG_PHYS_SHIFT
> >>+Architectures: arm64
> >>+Type: vm ioctl
> >>+Parameters: __u32 (in)
> >>+Returns: 0 on success, a negative value on error
> >>+
> >>+This ioctl is used to set the maximum physical address size for
> >>+the VM. The value is Log2(Maximum_Physical_Address). The value can only
> >>+be increased from the existing setting. The value cannot be changed
> >>+after the stage-2 page tables are allocated and will return an error.
> >>+
> >
> >Is there a way for userspace to discover what the underlying hardware
> >can actually support, beyond trial-and-error on this ioctl?
>
> Unfortunately, there is none. We don't expose ID_AA64MMFR0 via mrs emulation.
>

We should probably think about that. Perhaps it could be tied to the
return value of KVM_CAP_ARM_CONFIG_PHYS_SHIFT ?

> >>+static inline int kvm_reconfig_stage2(struct kvm *kvm, u32 phys_shift)
> >>+{
> >>+ int rc = 0;
> >>+ unsigned int pa_max, parange;
> >>+
> >>+ parange = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1) & 7;
> >>+ pa_max = id_aa64mmfr0_parange_to_phys_shift(parange);
> >>+ /* Raise it to 40bits for backward compatibility */
> >>+ pa_max = (pa_max < 40) ? 40 : pa_max;
> >>+ /* Make sure the size is supported/available */
> >>+ if (phys_shift > PHYS_MASK_SHIFT || phys_shift > pa_max)
> >>+ return -EINVAL;
> >>+ /*
> >>+ * The stage2 PGD is dependent on the settings we initialise here
> >>+ * and should be allocated only after this step. We cannot allow
> >>+ * down sizing the IPA size as there could be devices or memory
> >>+ * regions, that depend on the previous size.
> >>+ */
> >>+ mutex_lock(&kvm->slots_lock);
> >>+ if (kvm->arch.pgd || phys_shift < kvm->arch.phys_shift) {
> >>+ rc = -EPERM;
> >>+ } else if (phys_shift > kvm->arch.phys_shift) {
> >>+ kvm->arch.phys_shift = phys_shift;
> >>+ kvm->arch.s2_levels = stage2_pt_levels(kvm->arch.phys_shift);
> >>+ kvm->arch.vtcr_private = VTCR_EL2_SL0(kvm->arch.s2_levels) |
> >>+ TCR_T0SZ(kvm->arch.phys_shift);
> >>+ }
> >
> >I think you can rework the above to make it more obvious what's going on
> >in this way:
> >
> > rc = -EPERM;
> > if (kvm->arch.pgd || phys_shift < kvm->arch.phys_shift)
> > goto out_unlock;
> >
> > rc = 0;
> > if (phys_shift == kvm->arch.phys_shift)
> > goto out_unlock;
> >
> > kvm->arch.phys_shift = phys_shift;
> > kvm->arch.s2_levels = stage2_pt_levels(kvm->arch.phys_shift);
> > kvm->arch.vtcr_private = VTCR_EL2_SL0(kvm->arch.s2_levels) |
> > TCR_T0SZ(kvm->arch.phys_shift);
> >
> >out_unlock:
> >
>
> Sure.
>
>
>
> >>--- a/virt/kvm/arm/arm.c
> >>+++ b/virt/kvm/arm/arm.c
> >>@@ -1136,7 +1136,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
> >> return 0;
> >> }
> >> default:
> >>- return -EINVAL;
> >>+ return kvm_arch_dev_vm_ioctl(kvm, ioctl, arg);
> >> }
> >> }
> >>--
> >>2.13.6
> >>
> >
> >Have you considered making this capability a generic capability and
> >encoding this in the 'type' argument to KVM_CREATE_VM? This would
> >significantly simplify all the above and would allow you to drop patch 8
> >and 9 I think.
>
> No. I will take a look. Btw, there were patches flying around to support
> "userspace" requesting specific values for ID feature registers. But even that
> doesn't help with the detection part. May be that is another way to configure
> the size, but not sure about the current status of that work.
>

It's a bit stranded. Drew was driving this work (on cc). But the ID
register exposed to the guest should represent the actual limits
of the VM, so I don't think we need userspace to configure this, but we
can implement this in KVM based on the PA range configured for the VM.

Thanks,
-Christoffer

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