Re: [PATCH v14 02/21] KVM: Rename CONFIG_KVM_GENERIC_PRIVATE_MEM to CONFIG_KVM_GENERIC_GMEM_POPULATE

[Fuad Tabba <tabba@google.com>]

Hi Xiaoyao,

On Wed, 16 Jul 2025 at 13:02, Xiaoyao Li <xiaoyao.li@intel.com> wrote:
>
> On 7/16/2025 7:15 PM, David Hildenbrand wrote:
> > On 16.07.25 13:05, Fuad Tabba wrote:
> >> On Wed, 16 Jul 2025 at 12:02, Xiaoyao Li <xiaoyao.li@intel.com> wrote:
> >>>
> >>> On 7/16/2025 6:25 PM, David Hildenbrand wrote:
> >>>> On 16.07.25 10:31, Xiaoyao Li wrote:
> >>>>> On 7/16/2025 4:11 PM, Fuad Tabba wrote:
> >>>>>> On Wed, 16 Jul 2025 at 05:09, Xiaoyao Li<xiaoyao.li@intel.com> wrote:
> >>>>>>> On 7/15/2025 5:33 PM, Fuad Tabba wrote:
> >>>>>>>> The original name was vague regarding its functionality. This
> >>>>>>>> Kconfig
> >>>>>>>> option specifically enables and gates the kvm_gmem_populate()
> >>>>>>>> function,
> >>>>>>>> which is responsible for populating a GPA range with guest data.
> >>>>>>> Well, I disagree.
> >>>>>>>
> >>>>>>> The config KVM_GENERIC_PRIVATE_MEM was introduced by commit
> >>>>>>> 89ea60c2c7b5
> >>>>>>> ("KVM: x86: Add support for "protected VMs" that can utilize private
> >>>>>>> memory"), which is a convenient config for vm types that requires
> >>>>>>> private memory support, e.g., SNP, TDX, and KVM_X86_SW_PROTECTED_VM.
> >>>>>>>
> >>>>>>> It was commit e4ee54479273 ("KVM: guest_memfd: let
> >>>>>>> kvm_gmem_populate()
> >>>>>>> operate only on private gfns") that started to use
> >>>>>>> CONFIG_KVM_GENERIC_PRIVATE_MEM gates kvm_gmem_populate()
> >>>>>>> function. But
> >>>>>>> CONFIG_KVM_GENERIC_PRIVATE_MEM is not for kvm_gmem_populate() only.
> >>>>>>>
> >>>>>>> If using CONFIG_KVM_GENERIC_PRIVATE_MEM to gate
> >>>>>>> kvm_gmem_populate() is
> >>>>>>> vague and confusing, we can introduce KVM_GENERIC_GMEM_POPULATE
> >>>>>>> to gate
> >>>>>>> kvm_gmem_populate() and select KVM_GENERIC_GMEM_POPULATE under
> >>>>>>> CONFIG_KVM_GENERIC_PRIVATE_MEM.
> >>>>>>>
> >>>>>>> Directly replace CONFIG_KVM_GENERIC_PRIVATE_MEM with
> >>>>>>> KVM_GENERIC_GMEM_POPULATE doesn't look correct to me.
> >>>>>> I'll quote David's reply to an earlier version of this patch [*]:
> >>>>>
> >>>>> It's not related to my concern.
> >>>>>
> >>>>> My point is that CONFIG_KVM_GENERIC_PRIVATE_MEM is used for selecting
> >>>>> the private memory support. Rename it to KVM_GENERIC_GMEM_POPULATE is
> >>>>> not correct.
> >>>>
> >>>> It protects a function that is called kvm_gmem_populate().
> >>>>
> >>>> Can we stop the nitpicking?
> >>>
> >>> I don't think it's nitpicking.
> >>>
> >>> Could you loot into why it was named as KVM_GENERIC_PRIVATE_MEM in the
> >>> first place, and why it was picked to protect kvm_gmem_populate()?
> >>
> >> That is, in part, the point of this patch. This flag protects
> >> kvm_gmem_populate(), and the name didn't reflect that. Now it does. It
> >> is the only thing it protects.
> >
> > I'll note that the kconfig makes it clear that it depends on
> > KVM_GENERIC_MEMORY_ATTRIBUTES -- having support for private memory.
> >
> > In any case, CONFIG_KVM_GENERIC_PRIVATE_MEM is a bad name: what on earth
> > is generic private memory.
>
> "gmem" + "memory_attribute" is the generic private memory.
>
> If KVM_GENERIC_PRIVATE_MEM is a bad name, we can drop it, but not rename
> it to CONFIG_KVM_GENERIC_GMEM_POPULATE.
>
> > If CONFIG_KVM_GENERIC_GMEM_POPULATE is for some reason I don't
> > understand yet not the right name, can we have something that better
> > expresses that is is about KVM .. GMEM ... and POPULATE?
>
> I'm not objecting the name of CONFIG_KVM_GENERIC_GMEM_POPULATE, but
> objecting the simple rename. Does something below look reasonable?

Isn't the below practically the same as having one patch that adds
KVM_GENERIC_GMEM_POPULATE, followed by a patch that drops
KVM_GENERIC_PRIVATE_MEM?

Cheers,
/fuad


> ---
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 2eeffcec5382..3f87dcaaae83 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -135,6 +135,7 @@ config KVM_INTEL_TDX
>          bool "Intel Trust Domain Extensions (TDX) support"
>          default y
>          depends on INTEL_TDX_HOST
> +       select KVM_GENERIC_GMEM_POPULATE
>          help
>            Provides support for launching Intel Trust Domain Extensions
> (TDX)
>            confidential VMs on Intel processors.
> @@ -158,6 +159,7 @@ config KVM_AMD_SEV
>          depends on CRYPTO_DEV_SP_PSP && !(KVM_AMD=y && CRYPTO_DEV_CCP_DD=m)
>          select ARCH_HAS_CC_PLATFORM
>          select KVM_GENERIC_PRIVATE_MEM
> +       select KVM_GENERIC_GMEM_POPULATE
>          select HAVE_KVM_ARCH_GMEM_PREPARE
>          select HAVE_KVM_ARCH_GMEM_INVALIDATE
>          help
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 755b09dcafce..359baaae5e9f 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2556,7 +2556,7 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>   int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> int max_order);
>   #endif
>
> -#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
> +#ifdef CONFIG_KVM_GENERIC_GMEM_POPULATE
>   /**
>    * kvm_gmem_populate() - Populate/prepare a GPA range with guest data
>    *
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 49df4e32bff7..9b37ca009a22 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -121,6 +121,10 @@ config KVM_GENERIC_PRIVATE_MEM
>          select KVM_GMEM
>          bool
>
> +config KVM_GENERIC_GMEM_POPULATE
> +       bool
> +       depends on KVM_GMEM && KVM_GENERIC_MEMORY_ATTRIBUTES
> +
>   config HAVE_KVM_ARCH_GMEM_PREPARE
>          bool
>          depends on KVM_GMEM
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b2aa6bf24d3a..befea51bbc75 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -638,7 +638,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct
> kvm_memory_slot *slot,
>   }
>   EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);
>
> -#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
> +#ifdef CONFIG_KVM_GENERIC_GMEM_POPULATE
>   long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user
> *src, long npages,
>                         kvm_gmem_populate_cb post_populate, void *opaque)
>   {
>
>