Subject: Re: [PATCH] KVM: Add Kconfig option to signal cross-endian guests

From: Paolo Bonzini <[email-masked]>
Date: Thu, 9 Jul 2015 14:57:33 +0200

On 09/07/2015 11:48, Laurent Vivier wrote:
> 
> 
> On 09/07/2015 09:49, Thomas Huth wrote:
>> The option for supporting cross-endianness legacy guests in
>> the vhost and tun code should only be available on systems
>> that support cross-endian guests.
> 
> I'm sure I misunderstand something, but what happens if we use QEMU with
> TCG instead of KVM, i.e. a big endian powerpc kernel guest on x86_64
> little endian host ?

TCG does not yet support irqfd/ioeventfd, so it cannot be used with vhost.

Paolo

> Do you forbid the use of vhost in this case ?
> 
>> Signed-off-by: Thomas Huth <[email-masked]>
>> ---
>>  arch/arm/kvm/Kconfig     | 1 +
>>  arch/arm64/kvm/Kconfig   | 1 +
>>  arch/powerpc/kvm/Kconfig | 1 +
>>  drivers/net/Kconfig      | 1 +
>>  drivers/vhost/Kconfig    | 1 +
>>  virt/kvm/Kconfig         | 3 +++
>>  6 files changed, 8 insertions(+)
>>
>> diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
>> index bfb915d..9d8f363 100644
>> --- a/arch/arm/kvm/Kconfig
>> +++ b/arch/arm/kvm/Kconfig
>> @@ -31,6 +31,7 @@ config KVM
>>  	select KVM_VFIO
>>  	select HAVE_KVM_EVENTFD
>>  	select HAVE_KVM_IRQFD
>> +	select KVM_CROSS_ENDIAN_GUESTS
>>  	depends on ARM_VIRT_EXT && ARM_LPAE && ARM_ARCH_TIMER
>>  	---help---
>>  	  Support hosting virtualized guest machines.
>> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
>> index bfffe8f..9af39fe 100644
>> --- a/arch/arm64/kvm/Kconfig
>> +++ b/arch/arm64/kvm/Kconfig
>> @@ -31,6 +31,7 @@ config KVM
>>  	select KVM_VFIO
>>  	select HAVE_KVM_EVENTFD
>>  	select HAVE_KVM_IRQFD
>> +	select KVM_CROSS_ENDIAN_GUESTS
>>  	---help---
>>  	  Support hosting virtualized guest machines.
>>  
>> diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
>> index 3caec2c..e028710 100644
>> --- a/arch/powerpc/kvm/Kconfig
>> +++ b/arch/powerpc/kvm/Kconfig
>> @@ -79,6 +79,7 @@ config KVM_BOOK3S_64_HV
>>  	select KVM_BOOK3S_HV_POSSIBLE
>>  	select MMU_NOTIFIER
>>  	select CMA
>> +	select KVM_CROSS_ENDIAN_GUESTS
>>  	---help---
>>  	  Support running unmodified book3s_64 guest kernels in
>>  	  virtual machines on POWER7 and PPC970 processors that have
>> diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
>> index c18f9e6..0c4ce47 100644
>> --- a/drivers/net/Kconfig
>> +++ b/drivers/net/Kconfig
>> @@ -261,6 +261,7 @@ config TUN
>>  config TUN_VNET_CROSS_LE
>>  	bool "Support for cross-endian vnet headers on little-endian kernels"
>>  	default n
>> +	depends on KVM_CROSS_ENDIAN_GUESTS
>>  	---help---
>>  	  This option allows TUN/TAP and MACVTAP device drivers in a
>>  	  little-endian kernel to parse vnet headers that come from a
>> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
>> index 533eaf0..4d8ae6b 100644
>> --- a/drivers/vhost/Kconfig
>> +++ b/drivers/vhost/Kconfig
>> @@ -35,6 +35,7 @@ config VHOST
>>  
>>  config VHOST_CROSS_ENDIAN_LEGACY
>>  	bool "Cross-endian support for vhost"
>> +	depends on KVM_CROSS_ENDIAN_GUESTS
>>  	default n
>>  	---help---
>>  	  This option allows vhost to support guests with a different byte
>> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
>> index e2c876d..cc7b28a 100644
>> --- a/virt/kvm/Kconfig
>> +++ b/virt/kvm/Kconfig
>> @@ -47,3 +47,6 @@ config KVM_GENERIC_DIRTYLOG_READ_PROTECT
>>  config KVM_COMPAT
>>         def_bool y
>>         depends on COMPAT && !S390
>> +
>> +config KVM_CROSS_ENDIAN_GUESTS
>> +       bool
>>