linux-kernel - Re: [PATCH v8 1/2] x86/mm: add .bss..decrypted section to hold shared variables

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Date:   Thu, 13 Sep 2018 21:14:47 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v8 1/2] x86/mm: add .bss..decrypted section to hold shared
 variables



On 9/13/18 6:24 PM, Thomas Gleixner wrote:
> On Thu, 13 Sep 2018, Brijesh Singh wrote:
>>  
>> +void __weak mem_encrypt_free_decrypted_mem(void) { }
>> +
>>  void __ref free_initmem(void)
>>  {
>>  	e820__reallocate_tables();
>>  
>> +	mem_encrypt_free_decrypted_mem();
>> +
>>  	free_kernel_image_pages(&__init_begin, &__init_end);
>>  }
>>  
>> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
>> index b2de398..718acdf 100644
>> --- a/arch/x86/mm/mem_encrypt.c
>> +++ b/arch/x86/mm/mem_encrypt.c
>> @@ -348,6 +348,16 @@ bool sev_active(void)
>>  EXPORT_SYMBOL(sev_active);
>>  
>>  /* Architecture __weak replacement functions */
>> +void __init mem_encrypt_free_decrypted_mem(void)
>> +{
>> +	if (mem_encrypt_active())
>> +		return;
> Why?

Hmm, I think we should be able to free the unused memory when memory
encryption is active.
In that case, since memory range was mapped C=0 so we should do
set_memory_encryptyed()
to change from C=0 -> C=1 before freeing it.

>> +
>> +	free_init_pages("unused decrypted",
>> +			(unsigned long)__start_bss_decrypted_unused,
>> +			(unsigned long)__end_bss_decrypted);
> Everything _AFTER_ __start_bss_decrypted_unused _IS_ unused and can be
> freed. No conditional nothing. It's unused in any case.
>
> Thanks,
>
> 	tglx
>
>