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

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Message-ID: <20120810181422.GA14892@amt.cnet>
Date:	Fri, 10 Aug 2012 15:14:22 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.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 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?

The initial objective was to fix a vm crash, can you explain that
initial problem?

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