Re: [PATCH v3 02/13] xen: harmonize return types of hypercall handlers

From: Juergen Gross <jgross@suse.com>
To: Julien Grall <julien@xen.org>, Jan Beulich <jbeulich@suse.com>
Cc: xen-devel@lists.xenproject.org,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Bertrand Marquis" <bertrand.marquis@arm.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"Christopher Clark" <christopher.w.clark@gmail.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>
Subject: Re: [PATCH v3 02/13] xen: harmonize return types of hypercall handlers
Date: Fri, 17 Dec 2021 12:12:29 +0100	[thread overview]
Message-ID: <a91217dc-8f97-2882-ce08-2a408654295e@suse.com> (raw)
In-Reply-To: <e41d26aa-9ef5-459a-c143-caf28e43c47c@xen.org>


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On 17.12.21 11:41, Julien Grall wrote:
> Hi Juergen,
> 
> On 17/12/2021 08:50, Juergen Gross wrote:
>> On 17.12.21 08:45, Jan Beulich wrote:
>>> On 17.12.2021 06:34, Juergen Gross wrote:
>>>> On 16.12.21 22:15, Stefano Stabellini wrote:
>>>>> On Thu, 16 Dec 2021, Stefano Stabellini wrote:
>>>>>> On Thu, 16 Dec 2021, Juergen Gross wrote:
>>>>>>> On 16.12.21 03:10, Stefano Stabellini wrote:
>>>>>>>> The case of XENMEM_maximum_ram_page is interesting but it is not a
>>>>>>>> problem in reality because the max physical address size is only 
>>>>>>>> 40-bit
>>>>>>>> for aarch32 guests, so 32-bit are always enough to return the 
>>>>>>>> highest
>>>>>>>> page in memory for 32-bit guests.
>>>>>>>
>>>>>>> You are aware that this isn't the guest's max page, but the host's?
>>>>>
>>>>> I can see now that you meant to say that, no matter what is the max
>>>>> pseudo-physical address supported by the VM, 
>>>>> XENMEM_maximum_ram_page is
>>>>> supposed to return the max memory page, which could go above the
>>>>> addressibility limit of the VM.
>>>>>
>>>>> So XENMEM_maximum_ram_page should potentially be able to return 
>>>>> (1<<44)
>>>>> even when called by an aarch32 VM, with max IPA 40-bit.
>>>>>
>>>>> I would imagine it could be useful if dom0 is 32-bit but domUs are
>>>>> 64-bit on a 64-bit hypervisor (which I think it would be a very rare
>>>>> configuration on ARM.)
>>>>>
>>>>> Then it looks like XENMEM_maximum_ram_page needs to be able to 
>>>>> return a
>>>>> value > 32-bit when called by a 32-bit guest.
>>>>>
>>>>> The hypercall ABI follows the ARM C calling convention, so a 64-bit
>>>>> value should be returned using r0 and r1. But looking at
>>>>> xen/arch/arm/traps.c:do_trap_hypercall, it doesn't seem it ever 
>>>>> sets r1
>>>>> today. Only r0 is set, so effectively we only support 32-bit return
>>>>> values on aarch32 and for aarch32 guests.
>>>>>
>>>>> In other words, today all hypercalls on ARM return 64-bit to 64-bit
>>>>> guests and 32-bit to 32-bit guests. Which in the case of memory_op is
>>>>> "technically" the correct thing to do because it matches the C
>>>>> declaration in xen/include/xen/hypercall.h:
>>>>>
>>>>> extern long
>>>>> do_memory_op(
>>>>>       unsigned long cmd,
>>>>>       XEN_GUEST_HANDLE_PARAM(void) arg);
>>>>>
>>>>> So...  I guess the conclusion is that on ARM do_memory_op should 
>>>>> return
>>>>> "long" although it is not actually enough for a correct implementation
>>>>> of XENMEM_maximum_ram_page for aarch32 guests ?
>>>>>
>>>>
>>>> Hence my suggestion to check the return value of _all_ hypercalls to be
>>>> proper sign extended int values for 32-bit guests. This would fix all
>>>> potential issues without silently returning truncated values.
>>>
>>> Are we absolutely certain we have no other paths left where a possibly
>>> large unsigned values might be returned? In fact while
>>> compat_memory_op() does the necessary saturation, I've never been fully
>>> convinced of this being the best way of dealing with things. The range
>>> of error indicators is much smaller than [-INT_MIN,-1], so almost
>>> double the range of effectively unsigned values could be passed back
>>> fine. (Obviously we can't change existing interfaces, so this mem-op
>>> will need to remain as is.)
>>
>> In fact libxenctrl tries do deal with this fact by wrapping a memory_op
>> for a 32-bit environment into a multicall. This will work fine for a
>> 32-bit Arm guest, as xen_ulong_t is a uint64 there.
>>
>> So do_memory_op should return long on Arm, yes. OTOH doing so will
>> continue to be a problem in case a 32-bit guest doesn't use the
>> multicall technique for handling possible 64-bit return values.
>>
>> So I continue to argue that on Arm the return value of a hypercall
>> should be tested to fit into 32 bits. 
> 
> It would make sense. But what would you return if the value doesn't fit?

I guess some errno value would be appropriate, like -EDOM, -ERANGE or
-E2BIG.

> 
>> The only really clean alternative
>> would be to have separate hypercall function classes for Arm 32- and
>> 64-bit guests (which still could share most of the functions by letting
>> those return "int"). This would allow to use the 64-bit variant even for
>> 32-bit guests in multicall (fine as the return field is 64-bit wide),
>> and a probably saturating compat version for the 32-bit guest direct
>> hypercall.
> 
> I am not entirely sure to understand this proposal. Can you clarify it?

1. In patch 5 modify the hypercall table by adding another column, so
    instead of:
    +table:           pv32     pv64     hvm32    hvm64    arm
    use:
    +table:           pv32     pv64     hvm32    hvm64    arm32    arm64

2. Let most of the hypercalls just return int instead of long:
    +rettype: do int

3. Have an explicit 64-bit variant of memory_op (the 32-bit one is the
    compat variant existing already):
    +rettype: do64 long
    +prefix: do64 PREFIX_hvm
    +memory_op(unsigned long cmd, void *arg)

4. Use the appropriate calls in each column:
    +memory_op         compat   do64     hvm      hvm      compat  do64

5. In the Arm hypercall trap handler do:
    if ( is_32bit_domain(current->domain) )
        call_handlers_arm32(...);
    else
        call_handlers_arm64(...);

6. In the multicall handler always do:
    call_handlers_arm64(...);


Juergen

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