linux-kernel - Re: [PATCH v5 00/12] KVM: introduce readonly memslot

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Date:	Tue, 14 Aug 2012 12:51:06 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v5 00/12] KVM: introduce readonly memslot

On Tue, Aug 14, 2012 at 05:00:33PM +0300, Avi Kivity wrote:
> On 08/10/2012 09:14 PM, Marcelo Tosatti wrote:
> > On Tue, Aug 07, 2012 at 05:47:15PM +0800, Xiao Guangrong wrote:
> >> Changelog:
> >> - introduce KVM_PFN_ERR_RO_FAULT instead of dummy page
> >> - introduce KVM_HVA_ERR_BAD and optimize error hva indicators
> >> 
> >> The test case can be found at:
> >> http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/00819/migrate-perf.tar.bz2
> >> 
> >> In current code, if we map a readonly memory space from host to guest
> >> and the page is not currently mapped in the host, we will get a fault-pfn
> >> and async is not allowed, then the vm will crash.
> >> 
> >> As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD
> >> to the guest, read access is happy for readonly memslot, write access on
> >> readonly memslot will cause KVM_EXIT_MMIO exit.
> > 
> > Memory slots whose QEMU mapping is write protected is supported
> > today, as long as there are no write faults.
> > 
> > What prevents the use of mmap(!MAP_WRITE) to handle read-only memslots
> > again?
> 
> Userspace may want to modify the ROM (for example, when programming a
> flash device).  It is also possible to map an hva range rw through one
> slot and ro through another.

Right, can do that with multiple userspace maps to the same anonymous 
memory region (see other email).

The bugs noticed should be fixed.

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