Re: [PATCH V3 (resend) 06/19] x86: Add a boot option to enable and disable the direct map

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.05.2024 11:20, Roger Pau Monné wrote:
> On Mon, May 13, 2024 at 01:40:33PM +0000, Elias El Yandouzi wrote:
>> --- a/docs/misc/xen-command-line.pandoc
>> +++ b/docs/misc/xen-command-line.pandoc
>> @@ -799,6 +799,18 @@ that enabling this option cannot guarantee anything 
>> beyond what underlying
>>  hardware guarantees (with, where available and known to Xen, respective
>>  tweaks applied).
>>  
>> +### directmap (x86)
>> +> `= <boolean>`
>> +
>> +> Default: `true`
>> +
>> +Enable or disable the directmap region in Xen.
> 
> Enable or disable fully populating the directmap region in Xen.

Elias, would you please take care to address earlier review comments
before sending a new version? This and ...

>> +
>> +By default, Xen creates the directmap region which maps physical memory
>                                                           ^ all?
>> +in that region. Setting this to no will sparsely populate the directmap,
> 
> "Setting this to no" => "Disabling this option will sparsely..."

... this is what I had already asked for in reply to v2, of course with
different wording.

>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -1517,6 +1517,8 @@ void asmlinkage __init noreturn __start_xen(unsigned 
>> long mbi_p)
>>      if ( highmem_start )
>>          xenheap_max_mfn(PFN_DOWN(highmem_start - 1));
>>  
>> +    printk("Booting with directmap %s\n", has_directmap() ? "on" : "off");
> 
> IMO this wants to be printed as part of the speculation mitigations, see
> print_details() in spec_ctrl.c

And not "on" / "off", but "full" / "sparse" (and word order changed accordingly)
perhaps.

>> --- a/xen/common/Kconfig
>> +++ b/xen/common/Kconfig
>> @@ -80,12 +80,29 @@ config HAS_PMAP
>>  config HAS_SCHED_GRANULARITY
>>      bool
>>  
>> +config HAS_SECRET_HIDING
>> +    bool
>> +
>>  config HAS_UBSAN
>>      bool
>>  
>>  config MEM_ACCESS_ALWAYS_ON
>>      bool
>>  
>> +config SECRET_HIDING
>> +    bool "Secret hiding"
>> +    depends on HAS_SECRET_HIDING
> 
> IMO 'SECRET_HIDING' is too generic, this wants a more specific name.
> Maybe SPARCE_DIRECTMAP or some such.

This is another aspect I had raised on v2 already. SPARSE_DIRECTMAP
would be fine with me.

Jan