linux-kernel - Re: [PATCH v2 01/10] KVM: Document KVM_MAP

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Date: Wed, 17 Apr 2024 13:56:20 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, 
	"seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>, 
	"federico.parola@...ito.it" <federico.parola@...ito.it>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>, 
	"michael.roth@....com" <michael.roth@....com>
Subject: Re: [PATCH v2 01/10] KVM: Document KVM_MAP_MEMORY ioctl

On Tue, Apr 16, 2024 at 1:27 AM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
> > +  EAGAIN     The region is only processed partially.  The caller should
> > +             issue the ioctl with the updated parameters when `size` > 0.
> > +  EINTR      An unmasked signal is pending.  The region may be processed
> > +             partially.

The common convention is to only return errno if no page was processed.

> > +KVM_MAP_MEMORY populates guest memory with the range, `base_address` in (L1)
> > +guest physical address(GPA) and `size` in bytes.  `flags` must be zero  It's
> > +reserved for future use.  When the ioctl returns, the input values are
> > updated
> > +to point to the remaining range.  If `size` > 0 on return, the caller should
> > +issue the ioctl with the updated parameters.
> > +
> > +Multiple vcpus are allowed to call this ioctl simultaneously.  It's not
> > +mandatory for all vcpus to issue this ioctl.  A single vcpu can suffice.
> > +Multiple vcpus invocations are utilized for scalability to process the
> > +population in parallel.  If multiple vcpus call this ioctl in parallel, it
> > may
> > +result in the error of EAGAIN due to race conditions.
> > +
> > +This population is restricted to the "pure" population without triggering
> > +underlying technology-specific initialization.  For example, CoCo-related
> > +operations won't perform.  In the case of TDX, this API won't invoke
> > +TDH.MEM.PAGE.ADD() or TDH.MR.EXTEND().  Vendor-specific uAPIs are required
> > for
> > +such operations.
>
> Probably don't want to have TDX bits in here yet. Since it's talking about what
> KVM_MAP_MEMORY is *not* doing, it can just be dropped.

Let's rewrite everything to be more generic:

+KVM_MAP_MEMORY populates guest memory in the page tables of a vCPU.
+When the ioctl returns, the input values are updated to point to the
+remaining range.  If `size` > 0 on return, the caller should
+issue the ioctl again with updated parameters.
+
+In some cases, multiple vCPUs might share the page tables.  In this
+case, if this ioctl is called in parallel for multiple vCPUs the
+ioctl might return with `size > 0`.
+
+The ioctl may not be supported for all VMs.  You may use
+`KVM_CHECK_EXTENSION` on the VM file descriptor to check if it is
+supported.
+
+`flags` must currently be zero.


Paolo