Re: [RFC PATCH 2/6] virtio: add option to restrict memory access under Xen

[Juergen Gross <jgross@suse.com>]

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On 18.04.22 21:11, Stefano Stabellini wrote:
> On Sun, 17 Apr 2022, Oleksandr wrote:
>> On 16.04.22 01:01, Stefano Stabellini wrote:
>>> On Thu, 14 Apr 2022, Oleksandr Tyshchenko wrote:
>>>> From: Juergen Gross <jgross@suse.com>
>>>>
>>>> In order to support virtio in Xen guests add a config option enabling
>>>> the user to specify whether in all Xen guests virtio should be able to
>>>> access memory via Xen grant mappings only on the host side.
>>>>
>>>> This applies to fully virtualized guests only, as for paravirtualized
>>>> guests this is mandatory.
>>>>
>>>> This requires to switch arch_has_restricted_virtio_memory_access()
>>>> from a pure stub to a real function on x86 systems (Arm systems are
>>>> not covered by now).
>>>>
>>>> Add the needed functionality by providing a special set of DMA ops
>>>> handling the needed grant operations for the I/O pages.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>> ---
>>>>    arch/x86/mm/init.c        |  15 ++++
>>>>    arch/x86/mm/mem_encrypt.c |   5 --
>>>>    arch/x86/xen/Kconfig      |   9 +++
>>>>    drivers/xen/Kconfig       |  20 ++++++
>>>>    drivers/xen/Makefile      |   1 +
>>>>    drivers/xen/xen-virtio.c  | 177
>>>> ++++++++++++++++++++++++++++++++++++++++++++++
>>>>    include/xen/xen-ops.h     |   8 +++
>>>>    7 files changed, 230 insertions(+), 5 deletions(-)
>>>>    create mode 100644 drivers/xen/xen-virtio.c
>>>>
>>>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>>>> index d8cfce2..526a3b2 100644
>>>> --- a/arch/x86/mm/init.c
>>>> +++ b/arch/x86/mm/init.c
>>>> @@ -8,6 +8,8 @@
>>>>    #include <linux/kmemleak.h>
>>>>    #include <linux/sched/task.h>
>>>>    +#include <xen/xen.h>
>>>> +
>>>>    #include <asm/set_memory.h>
>>>>    #include <asm/e820/api.h>
>>>>    #include <asm/init.h>
>>>> @@ -1065,3 +1067,16 @@ unsigned long max_swapfile_size(void)
>>>>    	return pages;
>>>>    }
>>>>    #endif
>>>> +
>>>> +#ifdef CONFIG_ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
>>>> +int arch_has_restricted_virtio_memory_access(void)
>>>> +{
>>>> +	if (IS_ENABLED(CONFIG_XEN_PV_VIRTIO) && xen_pv_domain())
>>>> +		return 1;
>>>> +	if (IS_ENABLED(CONFIG_XEN_HVM_VIRTIO_GRANT) && xen_hvm_domain())
>>>> +		return 1;
>>> I think these two checks could be moved to a separate function in a Xen
>>> header, e.g. xen_restricted_virtio_memory_access, and here you could
>>> just
>>>
>>> if (xen_restricted_virtio_memory_access())
>>>       return 1;
>>
>> Agree, will do
>>
>>
>>>
>>>
>>>
>>>> +	return cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
>>>> +#endif
>>>> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
>>>> index 50d2099..dda020f 100644
>>>> --- a/arch/x86/mm/mem_encrypt.c
>>>> +++ b/arch/x86/mm/mem_encrypt.c
>>>> @@ -77,8 +77,3 @@ void __init mem_encrypt_init(void)
>>>>    	print_mem_encrypt_feature_info();
>>>>    }
>>>>    -int arch_has_restricted_virtio_memory_access(void)
>>>> -{
>>>> -	return cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
>>>> -}
>>>> -EXPORT_SYMBOL_GPL(arch_has_restricted_virtio_memory_access);
>>>> diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
>>>> index 85246dd..dffdffd 100644
>>>> --- a/arch/x86/xen/Kconfig
>>>> +++ b/arch/x86/xen/Kconfig
>>>> @@ -92,3 +92,12 @@ config XEN_DOM0
>>>>    	select X86_X2APIC if XEN_PVH && X86_64
>>>>    	help
>>>>    	  Support running as a Xen Dom0 guest.
>>>> +
>>>> +config XEN_PV_VIRTIO
>>>> +	bool "Xen virtio support for PV guests"
>>>> +	depends on XEN_VIRTIO && XEN_PV
>>>> +	default y
>>>> +	help
>>>> +	  Support virtio for running as a paravirtualized guest. This will
>>>> +	  need support on the backend side (qemu or kernel, depending on the
>>>> +	  virtio device types used).
>>>> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>>>> index 120d32f..fc61f7a 100644
>>>> --- a/drivers/xen/Kconfig
>>>> +++ b/drivers/xen/Kconfig
>>>> @@ -335,4 +335,24 @@ config XEN_UNPOPULATED_ALLOC
>>>>    	  having to balloon out RAM regions in order to obtain physical memory
>>>>    	  space to create such mappings.
>>>>    +config XEN_VIRTIO
>>>> +	bool "Xen virtio support"
>>>> +	default n
>>>> +	depends on VIRTIO && DMA_OPS
>>>> +	select ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
>>>> +	help
>>>> +	  Enable virtio support for running as Xen guest. Depending on the
>>>> +	  guest type this will require special support on the backend side
>>>> +	  (qemu or kernel, depending on the virtio device types used).
>>>> +
>>>> +config XEN_HVM_VIRTIO_GRANT
>>>> +	bool "Require virtio for fully virtualized guests to use grant
>>>> mappings"
>>>> +	depends on XEN_VIRTIO && X86_64
>>>> +	default y
>>>> +	help
>>>> +	  Require virtio for fully virtualized guests to use grant mappings.
>>>> +	  This will avoid the need to give the backend the right to map all
>>>> +	  of the guest memory. This will need support on the backend side
>>>> +	  (qemu or kernel, depending on the virtio device types used).
>>> I don't think we need 3 visible kconfig options for this.
>>>
>>> In fact, I would only add one: XEN_VIRTIO. We can have any X86 (or ARM)
>>> specific dependencies in the "depends" line under XEN_VIRTIO. And I
>>> don't think we need XEN_HVM_VIRTIO_GRANT as a kconfig option
>>> necessarely. It doesn't seem like some we want as build time option. At
>>> most, it could be a runtime option (like a command line) or a debug
>>> option (like an #define at the top of the source file.)
>>
>>
>> I don't know what was the initial idea of having and extra XEN_HVM_VIRTIO and
>> XEN_PV_VIRTIO options, but taking into the account that
>> they are only used in arch_has_restricted_virtio_memory_access() currently, I
>> share your opinion regarding a single XEN_VIRTIO option.
>>
>> Looking ahead (including changes in the commit #4), we can imagine the
>> resulting option:
>>
>> config XEN_VIRTIO
>>      bool "Xen virtio support"
>>      default n
>>      depends on VIRTIO && DMA_OPS
>>      depends on (X86_64 || ARM || ARM64)
>>      select ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
>>      help
>>        Enable virtio support for running as Xen guest. Depending on the
>>        guest type this will require special support on the backend side
>>        (qemu or kernel, depending on the virtio device types used).
>>
>>
>> and then arch_has_restricted_virtio_memory_access() per arch:
>>
>>
>> 1. x86:
>>
>> int arch_has_restricted_virtio_memory_access(void)
>> {
>>      return (xen_has_restricted_virtio_memory_access() ||
>>              cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT));
>> }
>>
>>
>> 2. Arm:
>>
>> int arch_has_restricted_virtio_memory_access(void)
>> {
>>      return xen_has_restricted_virtio_memory_access();
>> }
>>
>>
>> 3. xen.h:
>>
>> static inline int xen_has_restricted_virtio_memory_access(void)
>> {
>>      if (IS_ENABLED(CONFIG_XEN_VIRTIO) && (xen_pv_domain() ||
>> xen_hvm_domain()))
>>          return 1;
>>
>>      return 0;
>> }
>>
>>
>> Actually, as domain type on Arm is always XEN_HVM_DOMAIN, we could probably
>> have the following on Arm:
>>
>> int arch_has_restricted_virtio_memory_access(void)
>> {
>>      return IS_ENABLED(CONFIG_XEN_VIRTIO);
>> }
>>
>> but I would prefer not to diverge and use common
>> xen_has_restricted_virtio_memory_access().
>>
>> Any thoughts?
> 
> Yes, I would also prefer not to diverge between the x86 and arm versions
> of xen_has_restricted_virtio_memory_access. But what case are we trying
> to catch with (xen_pv_domain() || xen_hvm_domain()) ? Even on x86, it is
> not going to leave much out. Is it really meant only to exclude pvh
> domains?

It wouldn't exclude pvh domains.

> 
> I have the feeling that we could turn this check into:
> 
> static inline int xen_has_restricted_virtio_memory_access(void)
> {
>      return IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain();
> }
> 
> even on x86, but one of the xen/x86 maintainers should confirm.

I do confirm this is better and functionally equivalent.


Juergen

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