On Tue, 23 Feb 2021 16:46:50 +0100 Vitaly Kuznetsov <vkuzn...@redhat.com> wrote:
> Igor Mammedov <imamm...@redhat.com> writes: > > > On Mon, 22 Feb 2021 11:20:34 +0100 > > Vitaly Kuznetsov <vkuzn...@redhat.com> wrote: > > > >> Vitaly Kuznetsov <vkuzn...@redhat.com> writes: > >> > >> > Igor Mammedov <imamm...@redhat.com> writes: > >> > > >> >>> > >> >>> We need to distinguish because that would be sane. > >> >>> > >> >>> Enlightened VMCS is an extension to VMX, it can't be used without > >> >>> it. Genuine Hyper-V doesn't have a knob for enabling and disabling it, > >> >>> > >> >> ... > >> >>> That bein said, if > >> >>> guest CPU lacks VMX it is counter-productive to expose EVMCS. However, > >> >>> there is a problem with explicit enablement: what should > >> >>> > >> >>> 'hv-passthrough,hv-evmcs' option do? Just silently drop EVMCS? Doesn't > >> >>> sound sane to me. > >> >> based on above I'd error out is user asks for unsupported option > >> >> i.e. no VMX -> no hv-evmcs - if explicitly asked -> error out > >> > > >> > That's what I keep telling you but you don't seem to listen. 'Scratch > >> > CPU' can't possibly help with this use-case because when you parse > >> > > >> > 'hv-passthrough,hv-evmcs,vmx=off' you > >> > > >> > 1) "hv-passthrough" -> set EVMCS bit to '1' as it is supported by the > >> > host. > >> > > >> > 2) 'hv-evmcs' -> keep EVMCS bit '1' > >> > > >> > 3) 'vmx=off' -> you have no idea where EVMCS bit came from. > >> > > >> > We have to remember which options were aquired from the host and which > >> > were set explicitly by the user. > >> > >> Igor, > >> > >> could you please comment on the above? In case my line of thought is > >> correct, and it is impossible to distinguish between e.g. > >> > >> 'hv-passthrough,hv-evmcs,-vmx' > >> and > >> 'hv-passthrough,-vmx' > >> > >> without a custom parser (written just exactly the way I did in this > >> version, for example) regardless of when 'hv-passthrough' is > >> expanded. E.g. we have the exact same problem with > >> 'hv-default,hv-evmcs,-vmx'. I that case I see no point in discussing > > > > right, if we need to distinguish between explicit and implicit hv-evmcs set > > by > > hv-passthrough custom parser probably the way to go. > > > > However do we need actually need to do it? > > I think we really need that. See below ... > > > I'd treat 'hv-passthrough,-vmx' the same way as > > 'hv-passthrough,hv-evmcs,-vmx' > > and it applies not only hv-evmcs but other features hv-passthrough might set > > (i.e. if whatever was [un]set by hv-passthrough in combination with other > > features results in invalid config, QEMU shall error out instead of > > magically > > altering host provided hv-passthrough value). > > > > something like: > > 'hv-passthrough,-vmx' when hv-passthrough makes hv-evmcs bit set > > should result in > > error_setg(errp,"'vmx' feature can't be disabled when hv-evmcs is > > enabled," > > " either enable 'vmx' or disable 'hv-evmcs' along with > > disabling 'vmx'" > > > > making host's features set, *magically* mutable, depending on other user > > provided features > > is a bit confusing. One would never know what hv-passthrough actually > > means, and if > > enabling/disabling 'random' feature changes it. > > > > It's cleaner to do just what user asked (whether implicitly or explicitly) > > and error out > > in case it ends up in nonsense configuration. > > > > I don't seem to agree this is a sane behavior, especially if you replace > 'hv-passthrough' with 'hv-default' above. Removing 'vmx' from CPU for > Windows guests is common if you'd want to avoid nested configuration: > even without any Hyper-V guests created, Windows itself is a Hyper-V > partition. > > So a sane user will do: > > '-cpu host,hv-default,vmx=off' > > and on Intel he will get an error, and on AMD he won't. > > So what you're suggesting actually defeats the whole purpose of > 'hv-default' as upper-layer tools (think libvirt) will need to know that I'd assume it would be hard for libvirt to use 'hv-default' from migration point of view. It's semi opaque (one can find out what features it sets indirectly inspecting individual hv_foo features, and mgmt will need to know about them). If it will mutate when other features [un]set, upper layers might need to enumerate all these permutations to know which hosts are compatible or compare host feature sets every time before attempting migration. > Intel configurations for Windows guests are somewhat different. They'll > need to know what 'hv-evmcs' is. We're back to where we've started. we were talking about hv-passthrough, and if host advertises hv-evmcs QEMU should complain if user disabled features it depends on ( not silently fixing up configuration error). But the same applies to hv-default. > If we are to follow this approach let's just throw away 'hv-evmcs' from > 'hv-default' set, it's going to be much cleaner. But again, I don't > really believe it's the right way to go. if desired behavior, on Intel host for above config, to start without error then indeed defaults should not set 'hv-evmcs' if it results in invalid feature set.