Re: [PATCH V6 2/5] x86/hyper-v: Add hyperv Isolation VM check in the cc_platform_has()

[Tianyu Lan <ltykernel@gmail.com>]

On 12/10/2021 4:38 AM, Michael Kelley (LINUX) wrote:
> From: Tianyu Lan <ltykernel@gmail.com> Sent: Monday, December 6, 2021 11:56 PM
>>
>> Hyper-V provides Isolation VM which has memory encrypt support. Add
>> hyperv_cc_platform_has() and return true for check of GUEST_MEM_ENCRYPT
>> attribute.
>>
>> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
>> ---
>> Change since v3:
>> 	* Change code style of checking GUEST_MEM attribute in the
>> 	  hyperv_cc_platform_has().
>> ---
>>   arch/x86/kernel/cc_platform.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/x86/kernel/cc_platform.c b/arch/x86/kernel/cc_platform.c
>> index 03bb2f343ddb..47db88c275d5 100644
>> --- a/arch/x86/kernel/cc_platform.c
>> +++ b/arch/x86/kernel/cc_platform.c
>> @@ -11,6 +11,7 @@
>>   #include <linux/cc_platform.h>
>>   #include <linux/mem_encrypt.h>
>>
>> +#include <asm/mshyperv.h>
>>   #include <asm/processor.h>
>>
>>   static bool __maybe_unused intel_cc_platform_has(enum cc_attr attr)
>> @@ -58,9 +59,16 @@ static bool amd_cc_platform_has(enum cc_attr attr)
>>   #endif
>>   }
>>
>> +static bool hyperv_cc_platform_has(enum cc_attr attr)
>> +{
>> +	return attr == CC_ATTR_GUEST_MEM_ENCRYPT;
>> +}
>>
>>   bool cc_platform_has(enum cc_attr attr)
>>   {
>> +	if (hv_is_isolation_supported())
>> +		return hyperv_cc_platform_has(attr);
>> +
>>   	if (sme_me_mask)
>>   		return amd_cc_platform_has(attr);
>>
> 
> Throughout Linux kernel code, there are about 20 calls to cc_platform_has()
> with CC_ATTR_GUEST_MEM_ENCRYPT as the argument.  The original code
> (from v1 of this patch set) only dealt with the call in sev_setup_arch().   But
> with this patch, all the other calls that previously returned "false" will now
> return "true" in a Hyper-V Isolated VM.  I didn't try to analyze all these other
> calls, so I think there's an open question about whether this is the behavior
> we want.
> 

CC_ATTR_GUEST_MEM_ENCRYPT is for SEV support so far. Hyper-V Isolation
VM is based on SEV or software memory encrypt. Most checks can be 
reused. The difference is that SEV code use encrypt bit in the page
table to encrypt and decrypt memory while Hyper-V uses vTOM. But the sev
memory encrypt mask "sme_me_mask" is unset in the Hyper-V Isolation VM
where claims sev and sme are unsupported. The rest of checks for mem enc
bit are still safe. So reuse CC_ATTR_GUEST_MEM_ENCRYPT for Hyper-V.