linux-kernel - [PATCH v4 0/2] KVM: guest_memfd: use write for population

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Message-ID: <20250828153049.3922-1-kalyazin@amazon.com>
Date: Thu, 28 Aug 2025 15:30:50 +0000
From: "Kalyazin, Nikita" <kalyazin@...zon.co.uk>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "shuah@...nel.org"
	<shuah@...nel.org>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"michael.day@....com" <michael.day@....com>, "david@...hat.com"
	<david@...hat.com>, "jthoughton@...gle.com" <jthoughton@...gle.com>, "Roy,
 Patrick" <roypat@...zon.co.uk>, "Thomson, Jack" <jackabt@...zon.co.uk>,
	"Manwaring, Derek" <derekmn@...zon.com>, "Cali, Marco"
	<xmarcalx@...zon.co.uk>, "Kalyazin, Nikita" <kalyazin@...zon.co.uk>
Subject: [PATCH v4 0/2] KVM: guest_memfd: use write for population

[ based on kvm/next ]

Implement guest_memfd allocation and population via the write syscall.
This is useful in non-CoCo use cases where the host can access guest
memory.  Even though the same can also be achieved via userspace mapping
and memcpying from userspace, write provides a more performant option
because it does not need to set page tables and it does not cause a page
fault for every page like memcpy would.  Note that memcpy cannot be
accelerated via MADV_POPULATE_WRITE as it is not supported by
guest_memfd and relies on GUP.

Populating 512MiB of guest_memfd on a x86 machine:
 - via memcpy: 436 ms
 - via write:  202 ms (-54%)

v4:
 - Switch from implementing the write callback to write_iter
 - Remove conditional compilation
 - Rebase to kvm/next

v3:
 - https://lore.kernel.org/kvm/20250303130838.28812-1-kalyazin@amazon.com
 - David/Mike D: Only compile support for the write syscall if
   CONFIG_KVM_GMEM_SHARED_MEM (now gone) is enabled.
v2: 
 - https://lore.kernel.org/kvm/20241129123929.64790-1-kalyazin@amazon.com
 - Switch from an ioctl to the write syscall to implement population

v1:
 - https://lore.kernel.org/kvm/20241024095429.54052-1-kalyazin@amazon.com

Nikita Kalyazin (2):
  KVM: guest_memfd: add generic population via write
  KVM: selftests: update guest_memfd write tests

 .../testing/selftests/kvm/guest_memfd_test.c  | 85 +++++++++++++++++--
 virt/kvm/guest_memfd.c                        | 64 +++++++++++++-
 2 files changed, 142 insertions(+), 7 deletions(-)


base-commit: a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
-- 
2.50.1