linux-kernel - Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

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Message-ID: <Z2IXvsM0olS5GvbR@google.com>
Date: Tue, 17 Dec 2024 19:43:54 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Doug Covelli <doug.covelli@...adcom.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Zack Rusin <zack.rusin@...adcom.com>, 
	kvm <kvm@...r.kernel.org>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "the arch/x86 maintainers" <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...hat.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Joel Stanley <joel@....id.au>, Linux Doc Mailing List <linux-doc@...r.kernel.org>, linux-kernel@...r.kernel.org, 
	linux-kselftest <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

On Thu, Dec 12, 2024, Doug Covelli wrote:
> On Thu, Nov 14, 2024 at 10:45 AM Doug Covelli <doug.covelli@...adcom.com> wrote:
> > > For LINT1, it should be less performance critical; if it's possible
> > > to just go through all vCPUs, and do KVM_GET_LAPIC to check who you
> > > should send a KVM_NMI to, then I'd do that.  I'd also accept a patch
> > > that adds a VM-wide KVM_NMI ioctl that does the same in the hypervisor
> > > if it's useful for you.
> >
> > Thanks for the patch - I'll get it a try but it might not be right away.
> >
> > > And since I've been proven wrong already, what do you need INIT/SIPI for?
> >
> > I don't think this one is as critical.  I believe the reason it was
> > added was so that we can synchronize startup of the APs with execution
> > of the BSP for guests that do not do a good job of that (Windows).
> >
> > Doug
> 
> We were able to get the in-kernel APIC working with our code using the split
> IRQ chip option with our virtual EFI FW even w/o the traps for SVR and LVT0
> writes.  Performance of Windows VMs is greatly improved as expected.
> Unfortunately our ancient legacy BIOS will not work with > 1 VCPU due to lack
> of support for IPIs with an archaic delivery mode of remote read which it uses
> to discover APs by attempting to read their APIC ID register.  MSFT WHP supports
> this functionality via an option, WHvPartitionPropertyCodeApicRemoteReadSupport.
> 
> Changing our legacy BIOS is not an option so in order to support Windows VMs
> with the legacy BIOS with decent performance we would either need to add support
> for remote reads of the APIC ID register to KVM or support CR8 accesses w/o
> exiting w/o the in-kernel APIC in order.  Do you have a preference?

I didn't quite follow the CR8 access thing.  If the choice is between emulating
Remote Read IPIs and using a userspace local APIC, then I vote with both hands
for emulating Remote Reads, especially if we can do a half-assed version that
provides only what your crazy BIOS needs :-)

The biggest wrinkle I can think of is that KVM uses the Remote Read IPI encoding
for a paravirt vCPU kick feature, but I doubt that's used by Windows guests and
so can be sacrificed on the Altar of Ancient BIOS.