On 3/28/25 15:30, Lendacky, Thomas wrote:
> A page state change is typically followed by an access of the page(s) and
> results in another VMEXIT in order to map the page into the nested page
> table. Depending on the size of page state change request, this can
> generate a number of additional VMEXITs. For example, under SNP, when
> Linux is utilizing lazy memory acceptance, memory is typically accepted in
> 4M chunks. A page state change request is submitted to mark the pages as
> private, followed by validation of the memory. Since the guest_memfd
> currently only supports 4K pages, each page validation will result in
> VMEXIT to map the page, resulting in 1024 additional exits.
>
> When performing a page state change, invoke KVM_PRE_FAULT_MEMORY for the
> size of the page state change in order to pre-map the pages and avoid the
> additional VMEXITs. This helps speed up boot times.
Ping...
>
> Signed-off-by: Tom Lendacky <thomas.lenda...@amd.com>
> ---
> accel/kvm/kvm-all.c | 2 ++
> include/system/kvm.h | 1 +
> target/i386/kvm/kvm.c | 31 ++++++++++++++++++++++++++-----
> 3 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index f89568bfa3..0cd487cea7 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -93,6 +93,7 @@ bool kvm_allowed;
> bool kvm_readonly_mem_allowed;
> bool kvm_vm_attributes_allowed;
> bool kvm_msi_use_devid;
> +bool kvm_pre_fault_memory_supported;
> static bool kvm_has_guest_debug;
> static int kvm_sstep_flags;
> static bool kvm_immediate_exit;
> @@ -2732,6 +2733,7 @@ static int kvm_init(MachineState *ms)
> kvm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
> kvm_check_extension(s, KVM_CAP_USER_MEMORY2) &&
> (kvm_supported_memory_attributes & KVM_MEMORY_ATTRIBUTE_PRIVATE);
> + kvm_pre_fault_memory_supported = kvm_vm_check_extension(s,
> KVM_CAP_PRE_FAULT_MEMORY);
>
> if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
> s->kernel_irqchip_split = mc->default_kernel_irqchip_split ?
> ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
> diff --git a/include/system/kvm.h b/include/system/kvm.h
> index ab17c09a55..492ea8a383 100644
> --- a/include/system/kvm.h
> +++ b/include/system/kvm.h
> @@ -42,6 +42,7 @@ extern bool kvm_gsi_routing_allowed;
> extern bool kvm_gsi_direct_mapping;
> extern bool kvm_readonly_mem_allowed;
> extern bool kvm_msi_use_devid;
> +extern bool kvm_pre_fault_memory_supported;
>
> #define kvm_enabled() (kvm_allowed)
> /**
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 6c749d4ee8..7c39d30c5f 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -5999,9 +5999,11 @@ static bool host_supports_vmx(void)
> * because private/shared page tracking is already provided through other
> * means, these 2 use-cases should be treated as being mutually-exclusive.
> */
> -static int kvm_handle_hc_map_gpa_range(struct kvm_run *run)
> +static int kvm_handle_hc_map_gpa_range(X86CPU *cpu, struct kvm_run *run)
> {
> + struct kvm_pre_fault_memory mem;
> uint64_t gpa, size, attributes;
> + int ret;
>
> if (!machine_require_guest_memfd(current_machine))
> return -EINVAL;
> @@ -6012,13 +6014,32 @@ static int kvm_handle_hc_map_gpa_range(struct kvm_run
> *run)
>
> trace_kvm_hc_map_gpa_range(gpa, size, attributes, run->hypercall.flags);
>
> - return kvm_convert_memory(gpa, size, attributes &
> KVM_MAP_GPA_RANGE_ENCRYPTED);
> + ret = kvm_convert_memory(gpa, size, attributes &
> KVM_MAP_GPA_RANGE_ENCRYPTED);
> + if (ret || !kvm_pre_fault_memory_supported) {
> + return ret;
> + }
> +
> + /*
> + * Opportunistically pre-fault memory in. Failures are ignored so that
> any
> + * errors in faulting in the memory will get captured in KVM page fault
> + * path when the guest first accesses the page.
> + */
> + memset(&mem, 0, sizeof(mem));
> + mem.gpa = gpa;
> + mem.size = size;
> + while (mem.size) {
> + if (kvm_vcpu_ioctl(CPU(cpu), KVM_PRE_FAULT_MEMORY, &mem)) {
> + break;
> + }
> + }
> +
> + return 0;
> }
>
> -static int kvm_handle_hypercall(struct kvm_run *run)
> +static int kvm_handle_hypercall(X86CPU *cpu, struct kvm_run *run)
> {
> if (run->hypercall.nr == KVM_HC_MAP_GPA_RANGE)
> - return kvm_handle_hc_map_gpa_range(run);
> + return kvm_handle_hc_map_gpa_range(cpu, run);
>
> return -EINVAL;
> }
> @@ -6118,7 +6139,7 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run
> *run)
> break;
> #endif
> case KVM_EXIT_HYPERCALL:
> - ret = kvm_handle_hypercall(run);
> + ret = kvm_handle_hypercall(cpu, run);
> break;
> default:
> fprintf(stderr, "KVM: unknown exit reason %d\n", run->exit_reason);
>
> base-commit: 0f15892acaf3f50ecc20c6dad4b3ebdd701aa93e
