On 2/24/25 08:07, Yan Zhao wrote:
This series introduces a quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT as
suggested by Paolo and Sean [1].
The purpose of introducing this quirk is to allow KVM to honor guest PAT on
Intel platforms with self-snoop feature. This support was previously
reverted by commit 9d70f3fec144 ("Revert "KVM: VMX: Always honor guest PAT
on CPUs that support self-snoop"") due to a reported broken of an old bochs
driver which incorrectly set memory type to UC but did not expect that UC
would be very slow on certain Intel platforms.
Hi Yan,
the main issue with this series is that the quirk is not disabled only
for TDX VMs, but for *all* VMs if TDX is available.
There are two concepts here:
- which quirks can be disabled
- which quirks are active
I agree with making the first vendor-dependent, but for a different
reason: the new KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT must be hidden if
self-snoop is not present.
As to the second, we already have an example of a quirk that is also
active, though we don't represent that in kvm->arch.disabled_quirks:
that's KVM_X86_QUIRK_CD_NW_CLEARED which is for AMD only and is
effectively always disabled on Intel platforms. For those cases, we
need to expose the quirk anyway in KVM_CAP_DISABLE_QUIRKS2, so that
userspace knows that KVM is *aware* of a particular issue. In other
words, even if disabling it has no effect, userspace may want to know
that it can rely on the problematic behavior not being present.
I'm testing an alternative series and will post it shortly.
Paolo
Sean previously suggested to bottom out if the UC slowness issue is working
as intended so that we can enable the quirk only when the VMs are affected
by the old unmodifiable guests [2]. After consulting with CPU architects,
it's told that this behavior is expected on ICX/SPR Xeon platforms due to
the snooping implementation.
So, implement the quirk such that KVM enables it by default on all Intel
non-TDX platforms while having the quirk explicitly reference the old
unmodifiable guests that rely on KVM to force memory type to WB. Newer
userspace can disable the quirk by default and only leave it enabled if an
old unmodifiable guest is an concern.
The quirk is platform-specific valid, available only on Intel non-TDX
platforms. It is absent on Intel TDX and AMD platforms, where KVM always
honors guest PAT.
Patch 1 does the preparation of making quirks platform-specific valid.
Patch 2 makes the quirk to be present on Intel and absent on AMD.
Patch 3 makes the quirk to be absent on Intel TDX and self-snoop a hard
dependency to enable TDX [3].
As a new platform, TDX is always running on CPUs with self-snoop
feature. It has no worry to break old yet unmodifiable guests.
Simply have KVM always honor guest PAT on TDX enabled platforms.
Attaching/detaching non-coherent DMA devices would not lead to
mirrored EPTs being zapped for TDs then. A previous attempt for
this purpose is at [4].
This series is based on kvm-coco-queue. It was supposed to be included in
TDX's "the rest" section. We post it separately to start review earlier.
Patches 1 and 2 are changes to the generic code, which can also be applied
to kvm/queue. A proposal is to have them go into kvm/queue and we rebase on
that.
Patch 3 can be included in TDX's "the rest" section in the end.
Thanks
Yan
[1] https://lore.kernel.org/kvm/CABgObfa=t1dGR5cEhbUqVWTD03vZR4QrzEUgHxq+3JJ7YsA9pA@xxxxxxxxxxxxxx
[2] https://lore.kernel.org/kvm/Zt8cgUASZCN6gP8H@xxxxxxxxxx
[3] https://lore.kernel.org/kvm/ZuBSNS33_ck-w6-9@xxxxxxxxxx
[4] https://lore.kernel.org/kvm/20241115084600.12174-1-yan.y.zhao@xxxxxxxxx
Yan Zhao (3):
KVM: x86: Introduce supported_quirks for platform-specific valid
quirks
KVM: x86: Introduce Intel specific quirk
KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT
KVM: TDX: Always honor guest PAT on TDX enabled platforms
Documentation/virt/kvm/api.rst | 30 +++++++++++++++++++++++++
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/include/uapi/asm/kvm.h | 1 +
arch/x86/kvm/mmu.h | 2 +-
arch/x86/kvm/mmu/mmu.c | 14 +++++++-----
arch/x86/kvm/vmx/main.c | 1 +
arch/x86/kvm/vmx/tdx.c | 5 +++++
arch/x86/kvm/vmx/vmx.c | 39 +++++++++++++++++++++++++++------
arch/x86/kvm/x86.c | 7 +++---
arch/x86/kvm/x86.h | 12 +++++-----
10 files changed, 91 insertions(+), 22 deletions(-)
