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

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Date:	Sat, 11 Aug 2012 11:36:20 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.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 08/11/2012 02:14 AM, 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?
> 

It is happy to map !write host memory space to the readonly memslot,
and they can coexist as well.

readonly memslot checks the write-permission by seeing slot->flags and
!write memory checks the write-permission in hva_to_pfn() function
which checks vma->flags. It is no conflict.

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

The issue was trigged by this code:

                } else {
                        if (async && (vma->vm_flags & VM_WRITE))
                                *async = true;
                        pfn = KVM_PFN_ERR_FAULT;
                }

If the host memory region is readonly (!vma->vm_flags & VM_WRITE) and
its physical page is swapped out (or the file data does not be read in),
get_user_page_nowait will fail, above code reject to set async,
then we will get a fault pfn and async=false.

I guess this issue also exists in "QEMU write protected mapping" as
you mentioned above.

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