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

[Isaku Yamahata]

On Thu, Feb 29, 2024 at 01:36:29AM -0500,
Xiaoyao Li <xiaoyao.li@intel.com> 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
> +                */
> +                return 0;

The reference count leaks. Add memory_region_unref() is needed.

Otherwise looks good to me.
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
-- 
Isaku Yamahata <isaku.yamahata@linux.intel.com>