Re: [PATCH v8 08/13] KVM: guest_memfd: Allow host to map guest_memfd() pages

[David Hildenbrand <david@redhat.com>]

On 30.04.25 18:56, Fuad Tabba wrote:
> Add support for mmap() and fault() for guest_memfd backed memory
> in the host for VMs that support in-place conversion between
> shared and private. To that end, this patch adds the ability to
> check whether the VM type supports in-place conversion, and only
> allows mapping its memory if that's the case.
> 
> This patch introduces the configuration option KVM_GMEM_SHARED_MEM,
> which enables support for in-place shared memory.
> 
> It also introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
> indicates that the host can create VMs that support shared memory.
> Supporting shared memory implies that memory can be mapped when shared
> with the host.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   include/linux/kvm_host.h | 15 ++++++-
>   include/uapi/linux/kvm.h |  1 +
>   virt/kvm/Kconfig         |  5 +++
>   virt/kvm/guest_memfd.c   | 92 ++++++++++++++++++++++++++++++++++++++++
>   virt/kvm/kvm_main.c      |  4 ++
>   5 files changed, 116 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9419fb99f7c2..f3af6bff3232 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -729,6 +729,17 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
>   }
>   #endif
>   
> +/*
> + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
> + * private memory is enabled and it supports in-place shared/private conversion.
> + */
> +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)
> +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
> +{
> +	return false;
> +}
> +#endif
> +
>   #ifndef kvm_arch_has_readonly_mem
>   static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
>   {
> @@ -2516,7 +2527,9 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   
>   static inline bool kvm_mem_from_gmem(struct kvm *kvm, gfn_t gfn)
>   {
> -	/* For now, only private memory gets consumed from guest_memfd. */
> +	if (kvm_arch_gmem_supports_shared_mem(kvm))
> +		return true;

After our discussion yesterday, am I correct that we will not be 
querying the KVM capability, but instead the "SHARED_TRACKING" (or 
however that flag is called) on the underlying guest_memfd instead?

I assume the function would then look something like

if (!kvm_supports_gmem(kvm))
	return false;
if (kvm_arch_gmem_supports_shared_mem(kvm))
	return .. TBD, test the gmem flag for the slot via gfn
return kvm_mem_is_private(kvm, gfn);

> +
>   	return kvm_mem_is_private(kvm, gfn);
>   }
>   
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index b6ae8ad8934b..8bc8046c7f3a 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -930,6 +930,7 @@ struct kvm_enable_cap {
>   #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
>   #define KVM_CAP_X86_GUEST_MODE 238
>   #define KVM_CAP_ARM_WRITABLE_IMP_ID_REGS 239
> +#define KVM_CAP_GMEM_SHARED_MEM 240
>   
>   struct kvm_irq_routing_irqchip {
>   	__u32 irqchip;
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 559c93ad90be..f4e469a62a60 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -128,3 +128,8 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
>   config HAVE_KVM_ARCH_GMEM_INVALIDATE
>          bool
>          depends on KVM_GMEM
> +
> +config KVM_GMEM_SHARED_MEM
> +       select KVM_GMEM
> +       bool
> +       prompt "Enables in-place shared memory for guest_memfd"
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 6db515833f61..8bc8fc991d58 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -312,7 +312,99 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
>   	return gfn - slot->base_gfn + slot->gmem.pgoff;
>   }
>   
> +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> +/*
> + * Returns true if the folio is shared with the host and the guest.
> + */
> +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
> +{
> +	struct kvm_gmem *gmem = file->private_data;
> +
> +	/* For now, VMs that support shared memory share all their memory. */
> +	return kvm_arch_gmem_supports_shared_mem(gmem->kvm);

Similar here: likely we want to check the guest_memfd flag.

-- 
Cheers,

David / dhildenb