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KVM: Let userspace disable per-VM mem attributes, enable per-gmem attributes

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Allow the user to disable KVM_VM_MEMORY_ATTRIBUTES even when KVM supports
PRIVATE and SHARED attributes, and expose gmem_in_place_conversion as a
module parameter when per-VM attributes are supported.  I.e. let userspace
enable in-place PRIVATE<=>SHARED conversion of guest_memfd pages.

Provide both a Kconfig option and a (conditional) module param so that
deployments that use a custom kernel can fully disable per-VM tracking,
while not forcing distros to ship two separate kernels in order to provide
backwards compatibility for downstream users.

Don't allow running VMs with mixed tracking for a given instance of KVM,
i.e. disallow toggling the module param after KVM is loaded, as the extra
complexity needed to handle per-VM behavior far outweighs any potential
benefit.  E.g. neither TDX nor SNP supports live migration, so in effect
the requirement is that existing deployments that want to support both the
old and the new models would need to tell their VMM which flavor of
tracking to use.

[ Xiaoyao: Define module_param only if CONFIG_KVM_VM_MEMORY_ATTRIBUTES is enabled ]

Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260910-gmem-inplace-conversion-v13-22-dd6fbf94f4e1@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2026-09-10 16:55:48 -07:00
parent 31670c382e
commit 537ec2b15f
4 changed files with 42 additions and 6 deletions
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25
Documentation/admin-guide/kernel-parameters.txt
View File

@ -3156,6 +3156,31 @@ Kernel parameters
kvm.enable_vmware_backdoor=[KVM] Support VMware backdoor PV interface.
Default is false (don't support).
kvm.gmem_in_place_conversion=
[KVM] Controls whether KVM enables in-place conversion
support for guest_memfd and tracks the private/shared
state of memory per guest_memfd instead of per VM.
If enabled, KVM enables the KVM_SET_MEMORY_ATTRIBUTES2
ioctl on guest_memfd file descriptors and disables the
legacy VM-scoped KVM_SET_MEMORY_ATTRIBUTES ioctl for
private memory state tracking. Only the
KVM_MEMORY_ATTRIBUTE_PRIVATE attribute moves to
per-guest_memfd tracking; other attributes remain
per-VM.
This parameter toggles KVM's in-place conversion
capability support. Whether a VMM uses separate backends
or out-of-place memory management is determined by
userspace VMM design.
Note, this parameter is only available when
CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y. When
CONFIG_KVM_VM_MEMORY_ATTRIBUTES is not set, in-place
conversion is unconditionally enabled.
Default is N (off).
kvm.nx_huge_pages=
[KVM] Controls the software workaround for the
X86_BUG_ITLB_MULTIHIT bug.

4
arch/x86/include/asm/kvm_host.h
View File

@ -1851,7 +1851,9 @@ enum kvm_intr_type {
((vcpu) && (vcpu)->arch.handling_intr_from_guest && \
(!!in_nmi() == ((vcpu)->arch.handling_intr_from_guest == KVM_HANDLING_NMI)))
#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
#if defined(CONFIG_KVM_SW_PROTECTED_VM) || \
defined(CONFIG_KVM_INTEL_TDX) || \
defined(CONFIG_KVM_AMD_SEV)
#define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
#endif
#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT

14
arch/x86/kvm/Kconfig
View File

@ -81,13 +81,21 @@ config KVM_WERROR
If in doubt, say "N".
config KVM_VM_MEMORY_ATTRIBUTES
bool
bool "Enable per-VM PRIVATE vs. SHARED attributes (for CoCo VMs)"
depends on KVM_SW_PROTECTED_VM || KVM_INTEL_TDX || KVM_AMD_SEV
help
Enable support for tracking PRIVATE vs. SHARED memory using per-VM
memory attributes. Using per-VM attributes is deprecated in favor of
tracking PRIVATE state in guest_memfd. Select this if you need to run
CoCo VMs using a VMM that doesn't support guest_memfd memory
attributes.
If unsure, say N.
config KVM_SW_PROTECTED_VM
bool "Enable support for KVM software-protected VMs"
depends on EXPERT
depends on KVM_X86 && X86_64
select KVM_VM_MEMORY_ATTRIBUTES
help
Enable support for KVM software-protected VMs. Currently, software-
protected VMs are purely a development and testing vehicle for
@ -138,7 +146,6 @@ config KVM_INTEL_TDX
bool "Intel Trust Domain Extensions (TDX) support"
default y
depends on INTEL_TDX_HOST
select KVM_VM_MEMORY_ATTRIBUTES
select HAVE_KVM_ARCH_GMEM_POPULATE
help
Provides support for launching Intel Trust Domain Extensions (TDX)
@ -162,7 +169,6 @@ config KVM_AMD_SEV
depends on KVM_AMD && X86_64
depends on CRYPTO_DEV_SP_PSP && !(KVM_AMD=y && CRYPTO_DEV_CCP_DD=m)
select ARCH_HAS_CC_PLATFORM
select KVM_VM_MEMORY_ATTRIBUTES
select HAVE_KVM_ARCH_GMEM_CONVERT
select HAVE_KVM_ARCH_GMEM_RECLAIM
select HAVE_KVM_ARCH_GMEM_INVALIDATE

5
virt/kvm/kvm_main.c
View File

@ -103,7 +103,10 @@ static bool __ro_after_init allow_unsafe_mappings;
module_param(allow_unsafe_mappings, bool, 0444);
#ifdef kvm_arch_has_private_mem
bool __ro_after_init gmem_in_place_conversion = false;
bool __ro_after_init gmem_in_place_conversion = !IS_ENABLED(CONFIG_KVM_VM_MEMORY_ATTRIBUTES);
#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
module_param(gmem_in_place_conversion, bool, 0444);
#endif
EXPORT_SYMBOL_FOR_KVM_INTERNAL(gmem_in_place_conversion);
#endif