Re: [PATCH v2 rc] iommu/amd: Do not enable SNP when V2 page table is enabled

[Vasant Hegde <vashegde@amd.com>]

Tom,


On 4/9/2024 8:06 PM, Tom Lendacky wrote:
> On 4/9/24 00:13, Vasant Hegde wrote:
>> DTE[Mode]=0 is not supported when SNP is enabled in the host. That means
>> to support SNP, IOMMU must be configured with V1 page table (See IOMMU
>> spec [1] for the details). If user passes kernel command line to 
>> configure
>> IOMMU domains with v2 page table (amd_iommu=pgtbl_v2) then disable SNP.
> 
> Might be nice to say that you are doing as the user asked by not forcing 
> the pagetables to v1.

Sure. I will update the description.


> 
> One minor comment below, otherwise:
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> 
>>
>> [1] 
>> https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/specifications/48882_IOMMU.pdf
>>
>> Cc: Ashish Kalra <ashish.kalra@amd.com>
>> Cc: Michael Roth <michael.roth@amd.com>
>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
>> ---
>> Changes in v2:
>>    - Rebased on top of v6.9-rc3 and resolved the conflict.
>>
>> -Vasant
>>
>>   drivers/iommu/amd/init.c | 21 +++++++++++----------
>>   1 file changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>> index 005981c2f96a..0d9b69765099 100644
>> --- a/drivers/iommu/amd/init.c
>> +++ b/drivers/iommu/amd/init.c
>> @@ -3215,28 +3215,29 @@ static void iommu_snp_enable(void)
>>           return;
>>       /*
>>        * The SNP support requires that IOMMU must be enabled, and is
>> -     * not configured in the passthrough mode.
>> +     * configured with V1 page table (DTE[Mode] = 0 is not supported).
>>        */
>>       if (no_iommu || iommu_default_passthrough()) {
>>           pr_err("SNP: IOMMU disabled or configured in passthrough 
>> mode, SNP cannot be supported.\n");
>> -        cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
>> -        return;
>> +        goto disable_snp;
>> +    }
>> +
>> +    if (amd_iommu_pgtable != AMD_IOMMU_V1) {
>> +        pr_warn("SNP: IOMMU is configured with V2 page table mode, 
>> SNP cannot be supported.\n");
> 
> In the previous check, pr_err() is used, while here pr_warn() is used. 
> Should be consistent. Not sure if that means changing the pr_err() to 
> pr_warn() or making this pr_warn() a pr_err().

Looks like we had this inconsistency from beginning. May be we should 
convert it to warning. I will post separate fix for that.


-Vasant