From 7d7b9c7655a26e09c800ef40373078a80e90d9f3 Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Tue, 17 Sep 2024 18:00:49 +0200 Subject: target/i386: Exclude 'hv-syndbg' from 'hv-passthrough' Windows with Hyper-V role enabled doesn't boot with 'hv-passthrough' when no debugger is configured, this significantly limits the usefulness of the feature as there's no support for subtracting Hyper-V features from CPU flags at this moment (e.g. "-cpu host,hv-passthrough,-hv-syndbg" does not work). While this is also theoretically fixable, 'hv-syndbg' is likely very special and unneeded in the default set. Genuine Hyper-V doesn't seem to enable it either. Introduce 'skip_passthrough' flag to 'kvm_hyperv_properties' and use it as one-off to skip 'hv-syndbg' when enabling features in 'hv-passthrough' mode. Note, "-cpu host,hv-passthrough,hv-syndbg" can still be used if needed. As both 'hv-passthrough' and 'hv-syndbg' are debug features, the change should not have any effect on production environments. Signed-off-by: Vitaly Kuznetsov Link: https://lore.kernel.org/r/20240917160051.2637594-3-vkuznets@redhat.com Signed-off-by: Paolo Bonzini --- docs/system/i386/hyperv.rst | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) (limited to 'docs') diff --git a/docs/system/i386/hyperv.rst b/docs/system/i386/hyperv.rst index 2505dc4..009947e 100644 --- a/docs/system/i386/hyperv.rst +++ b/docs/system/i386/hyperv.rst @@ -262,14 +262,19 @@ Supplementary features ``hv-passthrough`` In some cases (e.g. during development) it may make sense to use QEMU in 'pass-through' mode and give Windows guests all enlightenments currently - supported by KVM. This pass-through mode is enabled by "hv-passthrough" CPU - flag. + supported by KVM. Note: ``hv-passthrough`` flag only enables enlightenments which are known to QEMU (have corresponding 'hv-' flag) and copies ``hv-spinlocks`` and ``hv-vendor-id`` values from KVM to QEMU. ``hv-passthrough`` overrides all other 'hv-' settings on - the command line. Also, enabling this flag effectively prevents migration as the - list of enabled enlightenments may differ between target and destination hosts. + the command line. + + Note: ``hv-passthrough`` does not enable ``hv-syndbg`` which can prevent certain + Windows guests from booting when used without proper configuration. If needed, + ``hv-syndbg`` can be enabled additionally. + + Note: ``hv-passthrough`` effectively prevents migration as the list of enabled + enlightenments may differ between target and destination hosts. ``hv-enforce-cpuid`` By default, KVM allows the guest to use all currently supported Hyper-V -- cgit v1.1 From 45f519950d4f70d092b552661323ef3c851efdf7 Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Tue, 17 Sep 2024 18:00:51 +0200 Subject: docs/system: Add recommendations to Hyper-V enlightenments doc While hyperv.rst already has all currently implemented Hyper-V enlightenments documented, it may be unclear what is the recommended set to achieve the best result. Add the corresponding section to the doc. Signed-off-by: Vitaly Kuznetsov Link: https://lore.kernel.org/r/20240917160051.2637594-5-vkuznets@redhat.com Signed-off-by: Paolo Bonzini --- docs/system/i386/hyperv.rst | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) (limited to 'docs') diff --git a/docs/system/i386/hyperv.rst b/docs/system/i386/hyperv.rst index 009947e..1c1de77 100644 --- a/docs/system/i386/hyperv.rst +++ b/docs/system/i386/hyperv.rst @@ -283,6 +283,36 @@ Supplementary features feature alters this behavior and only allows the guest to use exposed Hyper-V enlightenments. +Recommendations +--------------- + +To achieve the best performance of Windows and Hyper-V guests and unless there +are any specific requirements (e.g. migration to older QEMU/KVM versions, +emulating specific Hyper-V version, ...), it is recommended to enable all +currently implemented Hyper-V enlightenments with the following exceptions: + +- ``hv-syndbg``, ``hv-passthrough``, ``hv-enforce-cpuid`` should not be enabled + in production configurations as these are debugging/development features. +- ``hv-reset`` can be avoided as modern Hyper-V versions don't expose it. +- ``hv-evmcs`` can (and should) be enabled on Intel CPUs only. While the feature + is only used in nested configurations (Hyper-V, WSL2), enabling it for regular + Windows guests should not have any negative effects. +- ``hv-no-nonarch-coresharing`` must only be enabled if vCPUs are properly pinned + so no non-architectural core sharing is possible. +- ``hv-vendor-id``, ``hv-version-id-build``, ``hv-version-id-major``, + ``hv-version-id-minor``, ``hv-version-id-spack``, ``hv-version-id-sbranch``, + ``hv-version-id-snumber`` can be left unchanged, guests are not supposed to + behave differently when different Hyper-V version is presented to them. +- ``hv-crash`` must only be enabled if the crash information is consumed via + QAPI by higher levels of the virtualization stack. Enabling this feature + effectively prevents Windows from creating dumps upon crashes. +- ``hv-reenlightenment`` can only be used on hardware which supports TSC + scaling or when guest migration is not needed. +- ``hv-spinlocks`` should be set to e.g. 0xfff when host CPUs are overcommited + (meaning there are other scheduled tasks or guests) and can be left unchanged + from the default value (0xffffffff) otherwise. +- ``hv-avic``/``hv-apicv`` should not be enabled if the hardware does not + support APIC virtualization (Intel APICv, AMD AVIC). Useful links ------------ -- cgit v1.1