Re: [PATCH v3 10/70] kvm: handle KVM_EXIT_MEMORY_FAULT

[Daniel P . Berrangé]

On Wed, Nov 15, 2023 at 02:14:19AM -0500, Xiaoyao Li wrote:
> From: Chao Peng <chao.p.peng@linux.intel.com>
> 
> Currently only KVM_MEMORY_EXIT_FLAG_PRIVATE in flags is valid when
> KVM_EXIT_MEMORY_FAULT happens. It indicates userspace needs to do
> the memory conversion on the RAMBlock to turn the memory into desired
> attribute, i.e., private/shared.
> 
> 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.
> 
> 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>
> ---
>  accel/kvm/kvm-all.c | 76 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 66 insertions(+), 10 deletions(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 76e2404d54d2..58abbcb6926e 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2902,6 +2902,50 @@ 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;
> +    RAMBlock *rb;
> +    void *addr;
> +    int ret = -1;
> +
> +    section = memory_region_find(get_system_memory(), start, size);
> +    if (!section.mr) {
> +        return ret;
> +    }
> +
> +    if (memory_region_has_guest_memfd(section.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(section.mr) +
> +               section.offset_within_region;
> +        rb = qemu_ram_block_from_host(addr, false, &offset);
> +        /*
> +         * With KVM_SET_MEMORY_ATTRIBUTES by kvm_set_memory_attributes(),
> +         * operation on underlying file descriptor is only for releasing
> +         * unnecessary pages.
> +         */
> +        ram_block_convert_range(rb, offset, size, to_private);
> +    } else {
> +        warn_report("Convert non guest_memfd backed memory region "
> +                    "(0x%"HWADDR_PRIx" ,+ 0x%"HWADDR_PRIx") to %s",
> +                    start, size, to_private ? "private" : "shared");

Again, if you're returning '-1' to indicate error, then
using warn_report is wrong, it should be error_report.

warn_report is for when you return success, indicating
the problem was non-fatal.

> +    }
> +
> +    memory_region_unref(section.mr);
> +    return ret;
> +}
> +
>  int kvm_cpu_exec(CPUState *cpu)
>  {
>      struct kvm_run *run = cpu->kvm_run;
> @@ -2969,18 +3013,20 @@ 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);
> @@ -3067,6 +3113,16 @@ 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:
>              DPRINTF("kvm_arch_handle_exit\n");
>              ret = kvm_arch_handle_exit(cpu, run);
> -- 
> 2.34.1
> 

With regards,
Daniel
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