David Hildenbrand <david@xxxxxxxxxx> writes:
> 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@xxxxxxxxxx>
>> ---
>> 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);
>
Yes, I believe we're aligned here. I added a patch that will do this,
but it depends on other parts of the patch series and I think it's
better if you review it altogether when Fuad posts it (as opposed to
reviewing a snippet I drop here.
>> +
>> 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
