linux-kernel - Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently with VM word size

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Date:   Fri, 28 Oct 2022 16:31:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, mhal@...x.co
Subject: Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently
 with VM word size

On Fri, Oct 28, 2022, Paolo Bonzini wrote:
> On 10/27/22 19:22, Sean Christopherson wrote:
> > On Thu, Oct 27, 2022, Paolo Bonzini wrote:
> > > So, use the short size at activation time as well.  This means
> > > re-activating the cache if the guest requests the hypercall page
> > > multiple times with different word sizes (this can happen when
> > > kexec-ing, for example).
> > 
> > I don't understand the motivation for allowing a conditionally valid GPA.  I see
> > a lot of complexity and sub-optimal error handling for a use case that no one
> > cares about.  Realistically, userspace is never going to provide a GPA that only
> > works some of the time, because doing otherwise is just asking for a dead guest.
> 
> We _should_ be following the Xen API, which does not even say that the
> areas have to fit in a single page.

Ah, I didn't realize these are hypercall => userspace => ioctl() paths.

> In fact, even Linux's
> 
>         struct vcpu_register_runstate_memory_area area;
> 
>         area.addr.v = &per_cpu(xen_runstate, cpu);
>         if (HYPERVISOR_vcpu_op(VCPUOP_register_runstate_memory_area,
>                                xen_vcpu_nr(cpu), &area))
> 
> could fail or not just depending on the linker's whims, if I'm not
> very confused.
> 
> Other data structures *do* have to fit in a page, but the runstate area
> does not and it's exactly the one where the cache comes the most handy.
> For this I'm going to wait for David to answer.
> 
> That said, the whole gpc API is really messy 

No argument there.

> and needs to be cleaned up beyond what this series does.  For example,
> 
>         read_lock_irqsave(&gpc->lock, flags);
>         while (!kvm_gfn_to_pfn_cache_check(v->kvm, gpc, gpc->gpa,
>                                            sizeof(x))) {
>                 read_unlock_irqrestore(&gpc->lock, flags);
> 
>                 if (kvm_gfn_to_pfn_cache_refresh(v->kvm, gpc, gpc->gpa, sizeof(x)))
>                         return;
> 
>                 read_lock_irqsave(&gpc->lock, flags);
>         }
> 	...
>         read_unlock_irqrestore(&gpc->lock, flags);
> 
> should really be simplified to
> 
> 	khva = kvm_gpc_lock(gpc);
> 	if (IS_ERR(khva))
> 		return;
> 	...
> 	kvm_gpc_unlock(gpc);
> 
> Only the special preempt-notifier cases would have to use check/refresh
> by hand.  If needed they could even pass whatever length they want to
> __kvm_gpc_refresh with, explicit marking that it's a here-be-dragons __API.
> 
> Also because we're using the gpc from non-preemptible regions the rwlock
> critical sections should be enclosed in preempt_disable()/preempt_enable().
> Fortunately they're pretty small.
> 
> For now I think the best course of action is to quickly get the bugfix
> patches to Linus, and for 6.2 drop this one but otherwise keep the length
> in kvm_gpc_activate().

Works for me.