linux-kernel - Re: [PATCH v18 064/121] KVM: TDX: Create initial guest memory

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Date: Mon, 26 Feb 2024 10:50:55 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Isaku Yamahata <isaku.yamahata@...ux.intel.com>
Cc: David Matlack <dmatlack@...gle.com>, isaku.yamahata@...el.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com, 
	Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com, Sagi Shahar <sagis@...gle.com>, 
	Kai Huang <kai.huang@...el.com>, chen.bo@...el.com, hang.yuan@...el.com, 
	tina.zhang@...el.com, gkirkpatrick@...gle.com, 
	Vipin Sharma <vipinsh@...gle.com>
Subject: Re: [PATCH v18 064/121] KVM: TDX: Create initial guest memory

On Mon, Feb 26, 2024, Isaku Yamahata wrote:
> On Thu, Feb 01, 2024 at 03:06:46PM -0800,
> David Matlack <dmatlack@...gle.com> wrote:
> 
> > +Vipin Sharma
> > 
> > On Wed, Jan 31, 2024 at 4:21 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > On Mon, Jan 22, 2024, isaku.yamahata@...el.com wrote:
> > >
> > > The real reason for this drive-by pseudo-review is that I am hoping/wishing we
> > > can turn this into a generic KVM ioctl() to allow userspace to pre-map guest
> > > memory[*].
> > >
> > > If we're going to carry non-trivial code, we might as well squeeze as much use
> > > out of it as we can.
> > >
> > > Beyond wanting to shove this into KVM_MEMORY_ENCRYPT_OP, is there any reason why
> > > this is a VM ioctl() and not a vCPU ioctl()?  Very roughly, couldn't we use a
> > > struct like this as input to a vCPU ioctl() that maps memory, and optionally
> > > initializes memory from @source?
> > >
> > >         struct kvm_memory_mapping {
> > >                 __u64 base_gfn;
> > >                 __u64 nr_pages;
> > >                 __u64 flags;
> > >                 __u64 source;
> > >         }
> > >
> > > TDX would need to do special things for copying the source, but beyond that most
> > > of the code in this function is generic.
> > >
> > > [*] https://lore.kernel.org/all/65262e67-7885-971a-896d-ad9c0a760907@polito.it
> > 
> > We would also be interested in such an API to reduce the guest
> > performance impact of intra-host migration.
> 
> I introduce KVM_MEMORY_MAPPING and KVM_CAP_MEMORY_MAPPING with v19.

KVM_MEMORY_MAPPING is not a good ioctl() name.  There needs to be a verb in there
somewhere, e.g. KVM_MAP_MEMORY, KVM_FAULTIN_MEMORY, etc.

> We can continue the discussion there.

No, we absolutely cannot continue the conversation there.   That is not how kernel
development works.

Enough is enough.  I am archiving v19 and not touching it.

Please post an RFC for _just_ this functionality, and follow-up in existing,
pre-v19 conversations for anything else that changed between v18 and v19 and might
need additional input/discussion.