linux-kernel - Re: [PATCH] KVM: allow NULL writable argument to __kvm_faultin

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Message-ID: <Z36OnrAGOL9c7cku@google.com>
Date: Wed, 8 Jan 2025 06:41:34 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Christian Zigotzky <chzigotzky@...osoft.de>, linuxppc-dev@...ts.ozlabs.org, 
	regressions@...ts.linux.dev
Subject: Re: [PATCH] KVM: allow NULL writable argument to __kvm_faultin_pfn

On Mon, Jan 06, 2025, Sean Christopherson wrote:
> On Wed, Jan 01, 2025, Paolo Bonzini wrote:
> > kvm_follow_pfn() is able to work with NULL in the .map_writable field
> > of the homonymous struct.  But __kvm_faultin_pfn() rejects the combo
> > despite KVM for e500 trying to use it.  Indeed .map_writable is not
> > particularly useful if the flags include FOLL_WRITE and readonly
> > guest memory is not supported, so add support to __kvm_faultin_pfn()
> > for this case.
> 
> I would prefer to keep the sanity check to minimize the risk of a page fault
> handler not supporting opportunistic write mappings.  e500 is definitely the
> odd one out here.

Per a quick chat at PUCK, Paolo is going to try and fix the e500 code to actually
use the @writable param as it's intended.