Re: [PATCH v5 08/65] kvm: handle KVM_EXIT_MEMORY_FAULT

["Wang, Lei" <lei4.wang@intel.com>]

On 2/29/2024 14:36, Xiaoyao Li wrote:
> From: Chao Peng <chao.p.peng@linux.intel.com>
> 
> When geeting KVM_EXIT_MEMORY_FAULT exit, it indicates userspace needs to
> do the memory conversion on the RAMBlock to turn the memory into desired
> attribute, i.e., private/shared.
> 
> Currently only KVM_MEMORY_EXIT_FLAG_PRIVATE in flags is valid when
> KVM_EXIT_MEMORY_FAULT happens.
> 
> Note, KVM_EXIT_MEMORY_FAULT makes sense only when the RAMBlock has
> guest_memfd memory backend.
> 
> Note, KVM_EXIT_MEMORY_FAULT returns with -EFAULT, so special handling is
> added.
> 
> When page is converted from shared to private, the original shared
> memory can be discarded via ram_block_discard_range(). Note, shared
> memory can be discarded only when it's not back'ed by hugetlb because
> hugetlb is supposed to be pre-allocated and no need for discarding.
> 
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> 
> ---
> Changes in v4:
> - open-coded ram_block_discard logic;
> - change warn_report() to error_report(); (Daniel)
> ---
>  accel/kvm/kvm-all.c | 94 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 84 insertions(+), 10 deletions(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 70d482a2c936..87e4275932a7 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2903,6 +2903,68 @@ static void kvm_eat_signals(CPUState *cpu)
>      } while (sigismember(&chkset, SIG_IPI));
>  }
>  
> +static int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
> +{
> +    MemoryRegionSection section;
> +    ram_addr_t offset;
> +    MemoryRegion *mr;
> +    RAMBlock *rb;
> +    void *addr;
> +    int ret = -1;
> +
> +    if (!QEMU_PTR_IS_ALIGNED(start, qemu_host_page_size) ||
> +        !QEMU_PTR_IS_ALIGNED(size, qemu_host_page_size)) {
> +        return -1;
> +    }
> +
> +    if (!size) {
> +        return -1;
> +    }
> +
> +    section = memory_region_find(get_system_memory(), start, size);
> +    mr = section.mr;
> +    if (!mr) {
> +        return -1;
> +    }
> +
> +    if (memory_region_has_guest_memfd(mr)) {
> +        if (to_private) {
> +            ret = kvm_set_memory_attributes_private(start, size);
> +        } else {
> +            ret = kvm_set_memory_attributes_shared(start, size);
> +        }
> +
> +        if (ret) {
> +            memory_region_unref(section.mr);
> +            return ret;
> +        }
> +
> +        addr = memory_region_get_ram_ptr(mr) + section.offset_within_region;
> +        rb = qemu_ram_block_from_host(addr, false, &offset);
> +
> +        if (to_private) {
> +            if (rb->page_size != qemu_host_page_size) {
> +                /*
> +                * shared memory is back'ed by  hugetlb, which is supposed to be
> +                * pre-allocated and doesn't need to be discarded
> +                */

Nit: comment indentation is broken here.

> +                return 0;
> +            } else {
> +                ret = ram_block_discard_range(rb, offset, size);
> +            }
> +        } else {
> +            ret = ram_block_discard_guest_memfd_range(rb, offset, size);
> +        }
> +    } else {
> +        error_report("Convert non guest_memfd backed memory region "
> +                    "(0x%"HWADDR_PRIx" ,+ 0x%"HWADDR_PRIx") to %s",

Same as above.

> +                    start, size, to_private ? "private" : "shared");
> +    }
> +
> +    memory_region_unref(section.mr);
> +    return ret;
> +}
> +
>  int kvm_cpu_exec(CPUState *cpu)
>  {
>      struct kvm_run *run = cpu->kvm_run;
> @@ -2970,18 +3032,20 @@ int kvm_cpu_exec(CPUState *cpu)
>                  ret = EXCP_INTERRUPT;
>                  break;
>              }
> -            fprintf(stderr, "error: kvm run failed %s\n",
> -                    strerror(-run_ret));
> +            if (!(run_ret == -EFAULT && run->exit_reason == KVM_EXIT_MEMORY_FAULT)) {
> +                fprintf(stderr, "error: kvm run failed %s\n",
> +                        strerror(-run_ret));
>  #ifdef TARGET_PPC
> -            if (run_ret == -EBUSY) {
> -                fprintf(stderr,
> -                        "This is probably because your SMT is enabled.\n"
> -                        "VCPU can only run on primary threads with all "
> -                        "secondary threads offline.\n");
> -            }
> +                if (run_ret == -EBUSY) {
> +                    fprintf(stderr,
> +                            "This is probably because your SMT is enabled.\n"
> +                            "VCPU can only run on primary threads with all "
> +                            "secondary threads offline.\n");
> +                }
>  #endif
> -            ret = -1;
> -            break;
> +                ret = -1;
> +                break;
> +            }
>          }
>  
>          trace_kvm_run_exit(cpu->cpu_index, run->exit_reason);
> @@ -3064,6 +3128,16 @@ int kvm_cpu_exec(CPUState *cpu)
>                  break;
>              }
>              break;
> +        case KVM_EXIT_MEMORY_FAULT:
> +            if (run->memory_fault.flags & ~KVM_MEMORY_EXIT_FLAG_PRIVATE) {
> +                error_report("KVM_EXIT_MEMORY_FAULT: Unknown flag 0x%" PRIx64,
> +                             (uint64_t)run->memory_fault.flags);
> +                ret = -1;
> +                break;
> +            }
> +            ret = kvm_convert_memory(run->memory_fault.gpa, run->memory_fault.size,
> +                                     run->memory_fault.flags & KVM_MEMORY_EXIT_FLAG_PRIVATE);
> +            break;
>          default:
>              ret = kvm_arch_handle_exit(cpu, run);
>              break;