Re: [PATCH] x86/kvm: do not setup pv tlb flush when not paravirtualized

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Cc: pbonzini@redhat.com, sean.j.christopherson@intel.com,
	wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org,
	tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org
Subject: Re: [PATCH] x86/kvm: do not setup pv tlb flush when not paravirtualized
Date: Mon, 03 Feb 2020 10:59:10 +0100	[thread overview]
Message-ID: <87wo94ng9d.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200131155655.49812-1-cascardo@canonical.com>

Thadeu Lima de Souza Cascardo <cascardo@canonical.com> writes:

> kvm_setup_pv_tlb_flush will waste memory and print a misguiding message
> when KVM paravirtualization is not available.
>
> Intel SDM says that the when cpuid is used with EAX higher than the
> maximum supported value for basic of extended function, the data for the
> highest supported basic function will be returned.
>
> So, in some systems, kvm_arch_para_features will return bogus data,
> causing kvm_setup_pv_tlb_flush to detect support for pv tlb flush.
>
> Testing for kvm_para_available will work as it checks for the hypervisor
> signature.
>
> Besides, when the "nopv" command line parameter is used, it should not
> continue as well, as kvm_guest_init will no be called in that case.
>
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> ---
>  arch/x86/kernel/kvm.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 81045aabb6f4..d817f255aed8 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -736,6 +736,9 @@ static __init int kvm_setup_pv_tlb_flush(void)
>  {
>  	int cpu;
>  
> +	if (!kvm_para_available() || nopv)
> +		return 0;
> +
>  	if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
>  	    !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
>  	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {

The patch will fix the immediate issue, but why kvm_setup_pv_tlb_flush()
is just an arch_initcall() which will be executed regardless of the fact
if we are running on KVM or not?

In Hyper-V we setup PV TLB flush from ms_hyperv_init_platform() -- which
only happens if Hyper-V platform was detected. Why don't we do it from
kvm_init_platform() in KVM?

-- 
Vitaly