linux-kernel - Re: [PATCH v4 11/11] KVM: X86: Use common code for PV IPIs in linux guest

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Message-ID: <Zr9ZgDIhUEzUlOFa@google.com>
Date: Fri, 16 Aug 2024 06:52:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc: X86 Kernel <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Dave Hansen <dave.hansen@...el.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Xin Li <xin3.li@...el.com>, linux-perf-users@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Tony Luck <tony.luck@...el.com>, Andy Lutomirski <luto@...nel.org>, acme@...nel.org, 
	kan.liang@...ux.intel.com, Andi Kleen <andi.kleen@...el.com>, 
	Nikolay Borisov <nik.borisov@...e.com>, Sohil Mehta <sohil.mehta@...el.com>, 
	Zeng Guang <guang.zeng@...el.com>
Subject: Re: [PATCH v4 11/11] KVM: X86: Use common code for PV IPIs in linux guest

"x86/kvm:" for the scope.  "KVM: x86:" is for host-side KVM, this is guest code.

On Tue, Jul 09, 2024, Jacob Pan wrote:
> The paravirtual APIC hooks in KVM, some of which are used for sending PV
> IPIs, can reuse common code for ICR preparation. This shared code also
> encompasses NMI-source reporting when in effect.

Please state what the patch actually does, not what it can do.  For folks that
aren't intimately familiar with FRED (read: me), that second sentence in particular
is wildly unhelpful.  I had to download yet another version of the FRED spec, and
decipher the poorly documented software-defined encoding scheme introduced by this
series just to understand what this patch does.

And the order of patches in this series is broken.  Overloading the vector *before*
switching the PV IPI code to __prepare_ICR() will result in KVM sending garbage
to the host.  I.e. _all_ IPI implementations need to be made safe before the NMI
source reporting code can be introduced.

> Originally-by: Zeng Guang <guang.zeng@...el.com>
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
> v4: Refine comments, no functional change.
> ---
>  arch/x86/kernel/kvm.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 263f8aed4e2c..a45d60aa0302 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -516,15 +516,7 @@ static void __send_ipi_mask(const struct cpumask *mask, int vector)
>  
>  	local_irq_save(flags);
>  
> -	switch (vector) {
> -	default:
> -		icr = APIC_DM_FIXED | vector;
> -		break;
> -	case NMI_VECTOR:
> -		icr = APIC_DM_NMI;
> -		break;
> -	}
> -
> +	icr = __prepare_ICR(0, vector, 0);

Rather than force KVM to throw in junk dest+shorthand, what about adding a
__prepare_ICR_vector()?  Then KVM doesn't need to arbitrarily pass zeroes, and
even __prepare_ICR() itself benefits (IMO), e.g. this is nice and easy to read:

static inline unsigned int __prepare_ICR(unsigned int shortcut, int vector,
					 unsigned int dest)
{
	return shortcut | dest | __prepare_ICR_vector(vector);
}