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 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. 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. -- Vitaly
