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Date: Fri, 14 Jul 2023 11:57:15 +0300
From: Zhi Wang <zhi.wang.linux@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
Sagi Shahar <sagis@...gle.com>,
David Matlack <dmatlack@...gle.com>,
Kai Huang <kai.huang@...el.com>, chen.bo@...el.com,
linux-coco@...ts.linux.dev,
Chao Peng <chao.p.peng@...ux.intel.com>,
Ackerley Tng <ackerleytng@...gle.com>,
Vishal Annapurve <vannapurve@...gle.com>,
Michael Roth <michael.roth@....com>,
Yuan Yao <yuan.yao@...ux.intel.com>
Subject: Re: [RFC PATCH v3 08/11] KVM: Fix set_mem_attr ioctl when error
case
On Thu, 13 Jul 2023 15:03:54 -0700
Sean Christopherson <seanjc@...gle.com> wrote:
> On Wed, Jun 28, 2023, isaku.yamahata@...el.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> > kvm_vm_ioctl_set_mem_attributes() discarded an error code of xa_err()
> > unconditionally. If an error occurred at the beginning, return error.
> >
> > Fixes: 3779c214835b ("KVM: Introduce per-page memory attributes")
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> > ---
> > Changes v2 -> v3:
> > - Newly added
> > ---
> > virt/kvm/kvm_main.c | 12 ++++++++----
> > 1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 422d49634c56..fdef56f85174 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2423,6 +2423,7 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> > gfn_t start, end;
> > unsigned long i;
> > void *entry;
> > + int err = 0;
> >
> > /* flags is currently not used. */
> > if (attrs->flags)
> > @@ -2447,14 +2448,17 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> > KVM_MMU_UNLOCK(kvm);
> >
> > for (i = start; i < end; i++) {
> > - if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> > - GFP_KERNEL_ACCOUNT)))
> > + err = xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> > + GFP_KERNEL_ACCOUNT));
> > + if (err)
> > break;
> > }
> >
> > KVM_MMU_LOCK(kvm);
> > - if (i > start)
> > + if (i > start) {
> > + err = 0;
> > kvm_mem_attrs_changed(kvm, attrs->attributes, start, i);
> > + }
> > kvm_mmu_invalidate_end(kvm);
> > KVM_MMU_UNLOCK(kvm);
> >
> > @@ -2463,7 +2467,7 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> > attrs->address = i << PAGE_SHIFT;
> > attrs->size = (end - i) << PAGE_SHIFT;
> >
> > - return 0;
> > + return err;
>
> Aha! Idea (stolen from commit afb2acb2e3a3 ("KVM: Fix vcpu_array[0] races")).
> Rather than deal with a potential error partway through the updates, reserve all
> xarray entries head of time. That way the ioctl() is all-or-nothing, e.g. KVM
> doesn't need to update the address+size to capture progress, and userspace doesn't
> have to retry (which is probably pointless anyways since failure to allocate an
> xarray entry likely means the system/cgroup is under intense memory pressure).
>
> Assuming it works (compile tested only), I'll squash this:
>
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 46fbb4e019a6..8cb972038dab 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -2278,7 +2278,7 @@ struct kvm_s390_zpci_op {
>
> /* Available with KVM_CAP_MEMORY_ATTRIBUTES */
> #define KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES _IOR(KVMIO, 0xd2, __u64)
> -#define KVM_SET_MEMORY_ATTRIBUTES _IOWR(KVMIO, 0xd3, struct kvm_memory_attributes)
> +#define KVM_SET_MEMORY_ATTRIBUTES _IOW(KVMIO, 0xd3, struct kvm_memory_attributes)
>
> struct kvm_memory_attributes {
> __u64 address;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 9584491c0cd3..93e82e3f1e1f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2425,6 +2425,7 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> gfn_t start, end;
> unsigned long i;
> void *entry;
> + int r;
>
> /* flags is currently not used. */
> if (attrs->flags)
> @@ -2439,18 +2440,32 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> start = attrs->address >> PAGE_SHIFT;
> end = (attrs->address + attrs->size - 1 + PAGE_SIZE) >> PAGE_SHIFT;
>
> + if (WARN_ON_ONCE(start == end))
> + return -EINVAL;
> +
> entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
>
> mutex_lock(&kvm->slots_lock);
>
> + /*
> + * Reserve memory ahead of time to avoid having to deal with failures
> + * partway through setting the new attributes.
> + */
> + for (i = start; i < end; i++) {
> + r = xa_reserve(&kvm->mem_attr_array, i, GFP_KERNEL_ACCOUNT);
> + if (r)
> + goto out_unlock;
> + }
> +
> KVM_MMU_LOCK(kvm);
> kvm_mmu_invalidate_begin(kvm);
> kvm_mmu_invalidate_range_add(kvm, start, end);
> KVM_MMU_UNLOCK(kvm);
>
> for (i = start; i < end; i++) {
> - if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> - GFP_KERNEL_ACCOUNT)))
> + r = xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> + GFP_KERNEL_ACCOUNT));
> + if (KVM_BUG_ON(r, kvm))
> break;
> }
>
IIUC, If an error happenes here, we should bail out and call xa_release()?
Or the code below (which is not shown here) still changes the memory attrs
partially.
> @@ -2460,12 +2475,10 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> kvm_mmu_invalidate_end(kvm);
> KVM_MMU_UNLOCK(kvm);
>
> +out_unlock:
> mutex_unlock(&kvm->slots_lock);
>
> - attrs->address = i << PAGE_SHIFT;
> - attrs->size = (end - i) << PAGE_SHIFT;
> -
> - return 0;
> + return r;
> }
> #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */
>
> @@ -5078,9 +5091,6 @@ static long kvm_vm_ioctl(struct file *filp,
> goto out;
>
> r = kvm_vm_ioctl_set_mem_attributes(kvm, &attrs);
> -
> - if (!r && copy_to_user(argp, &attrs, sizeof(attrs)))
> - r = -EFAULT;
> break;
> }
> #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */
>
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