mirror of
https://github.com/torvalds/linux.git
synced 2024-11-22 04:02:20 +00:00
e1548088ff
Drop KVM's emulation of CR0.CD=1 on Intel CPUs now that KVM no longer honors guest MTRR memtypes, as forcing UC memory for VMs with non-coherent DMA only makes sense if the guest is using something other than PAT to configure the memtype for the DMA region. Furthermore, KVM has forced WB memory for CR0.CD=1 since commitfb279950ba("KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED"), and no known VMM in existence disables KVM_X86_QUIRK_CD_NW_CLEARED, let alone does so with non-coherent DMA. Lastly, commitfb279950ba("KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED") was from the same author as commitb18d5431ac("KVM: x86: fix CR0.CD virtualization"), and followed by a mere month. I.e. forcing UC memory was likely the result of code inspection or perhaps misdiagnosed failures, and not the necessitate by a concrete use case. Update KVM's documentation to note that KVM_X86_QUIRK_CD_NW_CLEARED is now AMD-only, and to take an erratum for lack of CR0.CD virtualization on Intel. Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com> Tested-by: Yongwei Ma <yongwei.ma@intel.com> Link: https://lore.kernel.org/r/20240309010929.1403984-3-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com>
68 lines
2.6 KiB
ReStructuredText
68 lines
2.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
=======================================
|
|
Known limitations of CPU virtualization
|
|
=======================================
|
|
|
|
Whenever perfect emulation of a CPU feature is impossible or too hard, KVM
|
|
has to choose between not implementing the feature at all or introducing
|
|
behavioral differences between virtual machines and bare metal systems.
|
|
|
|
This file documents some of the known limitations that KVM has in
|
|
virtualizing CPU features.
|
|
|
|
x86
|
|
===
|
|
|
|
``KVM_GET_SUPPORTED_CPUID`` issues
|
|
----------------------------------
|
|
|
|
x87 features
|
|
~~~~~~~~~~~~
|
|
|
|
Unlike most other CPUID feature bits, CPUID[EAX=7,ECX=0]:EBX[6]
|
|
(FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]13] (ZERO_FCS_FDS) are
|
|
clear if the features are present and set if the features are not present.
|
|
|
|
Clearing these bits in CPUID has no effect on the operation of the guest;
|
|
if these bits are set on hardware, the features will not be present on
|
|
any virtual machine that runs on that hardware.
|
|
|
|
**Workaround:** It is recommended to always set these bits in guest CPUID.
|
|
Note however that any software (e.g ``WIN87EM.DLL``) expecting these features
|
|
to be present likely predates these CPUID feature bits, and therefore
|
|
doesn't know to check for them anyway.
|
|
|
|
Nested virtualization features
|
|
------------------------------
|
|
|
|
TBD
|
|
|
|
x2APIC
|
|
------
|
|
When KVM_X2APIC_API_USE_32BIT_IDS is enabled, KVM activates a hack/quirk that
|
|
allows sending events to a single vCPU using its x2APIC ID even if the target
|
|
vCPU has legacy xAPIC enabled, e.g. to bring up hotplugged vCPUs via INIT-SIPI
|
|
on VMs with > 255 vCPUs. A side effect of the quirk is that, if multiple vCPUs
|
|
have the same physical APIC ID, KVM will deliver events targeting that APIC ID
|
|
only to the vCPU with the lowest vCPU ID. If KVM_X2APIC_API_USE_32BIT_IDS is
|
|
not enabled, KVM follows x86 architecture when processing interrupts (all vCPUs
|
|
matching the target APIC ID receive the interrupt).
|
|
|
|
MTRRs
|
|
-----
|
|
KVM does not virtualize guest MTRR memory types. KVM emulates accesses to MTRR
|
|
MSRs, i.e. {RD,WR}MSR in the guest will behave as expected, but KVM does not
|
|
honor guest MTRRs when determining the effective memory type, and instead
|
|
treats all of guest memory as having Writeback (WB) MTRRs.
|
|
|
|
CR0.CD
|
|
------
|
|
KVM does not virtualize CR0.CD on Intel CPUs. Similar to MTRR MSRs, KVM
|
|
emulates CR0.CD accesses so that loads and stores from/to CR0 behave as
|
|
expected, but setting CR0.CD=1 has no impact on the cachaeability of guest
|
|
memory.
|
|
|
|
Note, this erratum does not affect AMD CPUs, which fully virtualize CR0.CD in
|
|
hardware, i.e. put the CPU caches into "no fill" mode when CR0.CD=1, even when
|
|
running in the guest. |