Re: [PATCH v3 2/8] domain: update GADDR based runstate guest area

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.09.2023 11:44, Roger Pau Monné wrote:
> On Wed, May 03, 2023 at 05:55:11PM +0200, Jan Beulich wrote:
>> Before adding a new vCPU operation to register the runstate area by
>> guest-physical address, add code to actually keep such areas up-to-date.
>>
>> Note that updating of the area will be done exclusively following the
>> model enabled by VMASST_TYPE_runstate_update_flag for virtual-address
>> based registered areas.
>>
>> Note further that pages aren't marked dirty when written to (matching
>> the handling of space mapped by map_vcpu_info()), on the basis that the
>> registrations are lost anyway across migration (or would need re-
>> populating at the target for transparent migration). Plus the contents
>> of the areas in question have to be deemed volatile in the first place
>> (so saving a "most recent" value is pretty meaningless even for e.g.
>> snapshotting).
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>> ---
>> RFC: HVM guests (on x86) can change bitness and hence layout (and size!
>>      and alignment) of the runstate area. I don't think it is an option
>>      to require 32-bit code to pass a range such that even the 64-bit
>>      layout wouldn't cross a page boundary (and be suitably aligned). I
>>      also don't see any other good solution, so for now a crude approach
>>      with an extra boolean is used (using has_32bit_shinfo() isn't race
>>      free and could hence lead to overrunning the mapped space).
> 
> Shouldn't a well behaved guest attempt to unmap the runstate area
> before changing bitness?  I would expect this to happen for example
> when OVMF relinquishes control to the OS.

Well, the OVMF example falls in a different class: Before transferring
ownership of the system, it wants to unmap regardless of whether bitness
is going to change, or else memory corruption can occur to the following
entity.

Jan