Re: [PATCH v3 01/22] KVM: x86: Drop kvm_user_return_msr_cpu_online()

Yuan Yao
2022-09-05T05:30:47Z

On Thu, Sep 01, 2022 at 07:17:36PM -0700, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <seanjc@google.com>
>
> KVM/X86 uses user return notifier to switch MSR for guest or user space.
> Snapshot host values on CPU online, change MSR values for guest, and
> restore them on returning to user space. The current code abuses
> kvm_arch_hardware_enable() which is called on kvm module initialization or
> CPU online.
>
> Remove such the abuse of kvm_arch_hardware_enable() by capturing the host
> value on the first change of the MSR value to guest VM instead of CPU
> online.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>

Reviewed-by: Yuan Yao <yuan.yao@intel.com>

> ---
> arch/x86/kvm/x86.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 205ebdc2b11b..0e200fe44b35 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -196,6 +196,7 @@ module_param(eager_page_split, bool, 0644);
>
> struct kvm_user_return_msrs {
> struct user_return_notifier urn;
> + bool initialized;
> bool registered;
> struct kvm_user_return_msr_values {
> u64 host;
> @@ -409,18 +410,20 @@ int kvm_find_user_return_msr(u32 msr)
> }
> EXPORT_SYMBOL_GPL(kvm_find_user_return_msr);
>
> -static void kvm_user_return_msr_cpu_online(void)
> +static void kvm_user_return_msr_init_cpu(struct kvm_user_return_msrs *msrs)
> {
> - unsigned int cpu = smp_processor_id();
> - struct kvm_user_return_msrs *msrs = per_cpu_ptr(user_return_msrs, cpu);
> u64 value;
> int i;
>
> + if (msrs->initialized)
> + return;
> +
> for (i = 0; i < kvm_nr_uret_msrs; ++i) {
> rdmsrl_safe(kvm_uret_msrs_list[i], &value);
> msrs->values[i].host = value;
> msrs->values[i].curr = value;
> }
> + msrs->initialized = true;
> }
>
> int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
> @@ -429,6 +432,8 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
> struct kvm_user_return_msrs *msrs = per_cpu_ptr(user_return_msrs, cpu);
> int err;
>
> + kvm_user_return_msr_init_cpu(msrs);
> +
> value = (value & mask) | (msrs->values[slot].host & ~mask);
> if (value == msrs->values[slot].curr)
> return 0;
> @@ -9212,7 +9217,12 @@ int kvm_arch_init(void *opaque)
> return -ENOMEM;
> }
>
> - user_return_msrs = alloc_percpu(struct kvm_user_return_msrs);
> + /*
> + * __GFP_ZERO to ensure user_return_msrs.values[].initialized = false.
> + * See kvm_user_return_msr_init_cpu().
> + */
> + user_return_msrs = alloc_percpu_gfp(struct kvm_user_return_msrs,
> + GFP_KERNEL | __GFP_ZERO);
> if (!user_return_msrs) {
> printk(KERN_ERR "kvm: failed to allocate percpu kvm_user_return_msrs\n");
> r = -ENOMEM;
> @@ -11836,7 +11846,6 @@ int kvm_arch_hardware_enable(void)
> u64 max_tsc = 0;
> bool stable, backwards_tsc = false;
>
> - kvm_user_return_msr_cpu_online();
> ret = static_call(kvm_x86_hardware_enable)();
> if (ret != 0)
> return ret;
> --
> 2.25.1
>