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From: Andrea Bolognani <abologna@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: groug@kaod.org, aik@ozlabs.ru, qemu-ppc@nongnu.org,
	qemu-devel@nongnu.org, clg@kaod.org
Subject: Re: [Qemu-devel] [RFC for-2.13 0/7] spapr: Clean up pagesize handling
Date: Mon, 07 May 2018 15:48:54 +0200	[thread overview]
Message-ID: <679d506dbec32c4978a03fe07f21de3e43fba2c0.camel@redhat.com> (raw)
In-Reply-To: <20180427121700.GP8800@umbus.fritz.box>

On Fri, 2018-04-27 at 22:17 +1000, David Gibson wrote:
> On Fri, Apr 27, 2018 at 10:31:10AM +0200, Andrea Bolognani wrote:
> > On Fri, 2018-04-27 at 12:14 +1000, David Gibson wrote:
> > > Right.. note that with the draft qemu patches a TCG guest will be
> > > prevented from using hugepages *by default* (the default value of the
> > > capability is 16).  You have to explicitly change it to allow
> > > hugepages to be used in a TCG guest (but you don't have to supply
> > > hugepage backing).
> > 
> > ... this will already happen. That's okay[1], we can't really
> > avoid it if we want to ensure consistent behavior between KVM and
> > TCG.
> 
> So.. regarding [1].  The draft patches *do* change the behaviour on
> older machine types.  I'll consider revisiting that, but I'd need to
> be convinced.  Basically we have to choose between consistency between
> accelerator and consistency between versions.  I think the former is
> the better choice; at least I think it is given that we *can* get both
> for the overwhelmingly common case in production (KVM HV).

Forgot to answer this point.

I agree that consistency between accelerators is the sane option
going forward, but changing the behavior for old machine types will
cause existing guests which have been using hugepages to lose the
ability to do so after being restarted on a newer QEMU.

Isn't that exactly the kind of scenario versioned machine types are
supposed to prevent?

-- 
Andrea Bolognani / Red Hat / Virtualization

  reply	other threads:[~2018-05-07 13:49 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-19  6:29 [Qemu-devel] [RFC for-2.13 0/7] spapr: Clean up pagesize handling David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 1/7] spapr: Maximum (HPT) pagesize property David Gibson
2018-05-02 21:06   ` Murilo Opsfelder Araujo
2018-05-03  1:34     ` David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 2/7] spapr: Use maximum page size capability to simplify memory backend checking David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 3/7] target/ppc: Add ppc_hash64_filter_pagesizes() David Gibson
2018-05-03 15:57   ` Murilo Opsfelder Araujo
2018-05-04  6:30     ` David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 4/7] spapr: Add cpu_apply hook to capabilities David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 5/7] spapr: Limit available pagesizes to provide a consistent guest environment David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 6/7] spapr: Don't rewrite mmu capabilities in KVM mode David Gibson
2018-04-19  6:29 ` [Qemu-devel] [RFC for-2.13 7/7] spapr_pci: Remove unhelpful pagesize warning David Gibson
2018-04-19 15:30 ` [Qemu-devel] [RFC for-2.13 0/7] spapr: Clean up pagesize handling Andrea Bolognani
2018-04-20  2:35   ` David Gibson
2018-04-20  9:31     ` Andrea Bolognani
2018-04-20 10:21       ` David Gibson
2018-04-23  8:31         ` Andrea Bolognani
2018-04-24  1:26           ` David Gibson
2018-04-24 15:35         ` Andrea Bolognani
2018-04-25  6:32           ` David Gibson
2018-04-25 16:09         ` Andrea Bolognani
2018-04-26  0:55           ` David Gibson
2018-04-26  8:45             ` Andrea Bolognani
2018-04-27  2:14               ` David Gibson
2018-04-27  8:31                 ` Andrea Bolognani
2018-04-27 12:17                   ` David Gibson
2018-05-07 13:48                     ` Andrea Bolognani [this message]
2018-06-14  1:52                       ` David Gibson

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