linux-kernel - Re: [PATCH 1/2] KVM: x86: reduce pvclock_gtod_sync

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Date:   Wed, 7 Apr 2021 14:40:21 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, dwmw@...zon.co.uk
Subject: Re: [PATCH 1/2] KVM: x86: reduce pvclock_gtod_sync_lock critical
 sections

On Tue, Mar 30, 2021 at 12:59:57PM -0400, Paolo Bonzini wrote:
> There is no need to include changes to vcpu->requests into
> the pvclock_gtod_sync_lock critical section.  The changes to
> the shared data structures (in pvclock_update_vm_gtod_copy)
> already occur under the lock.
> 
> Cc: David Woodhouse <dwmw@...zon.co.uk>
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fe806e894212..0a83eff40b43 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2562,10 +2562,12 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
>  
>  	kvm_hv_invalidate_tsc_page(kvm);
>  
> -	spin_lock(&ka->pvclock_gtod_sync_lock);
>  	kvm_make_mclock_inprogress_request(kvm);
> +

Might be good to serialize against two kvm_gen_update_masterclock
callers? Otherwise one caller could clear KVM_REQ_MCLOCK_INPROGRESS,
while the other is still at pvclock_update_vm_gtod_copy().

Otherwise, looks good.