linux-kernel - Re: [PATCHv5 27/30] x86/kvm: Use bounce buffers for TD guest

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Message-ID: <65577d45-8b15-08b6-2de4-3ca820e4d7a3@amd.com>
Date:   Thu, 10 Mar 2022 08:29:01 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        brijesh.singh@....com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5 27/30] x86/kvm: Use bounce buffers for TD guest

On 3/9/22 14:07, Dave Hansen wrote:
> On 3/2/22 06:28, Kirill A. Shutemov wrote:
>> --- a/arch/x86/coco/tdx.c
>> +++ b/arch/x86/coco/tdx.c
>> @@ -5,6 +5,7 @@
>>   #define pr_fmt(fmt)     "tdx: " fmt
>>   
>>   #include <linux/cpufeature.h>
>> +#include <linux/swiotlb.h>
>>   #include <asm/coco.h>
>>   #include <asm/tdx.h>
>>   #include <asm/vmx.h>
>> @@ -627,5 +628,7 @@ void __init tdx_early_init(void)
>>   	x86_platform.guest.enc_tlb_flush_required = tdx_tlb_flush_required;
>>   	x86_platform.guest.enc_status_change_finish = tdx_enc_status_changed;
>>   
>> +	swiotlb_force = SWIOTLB_FORCE;
>> +
>>   	pr_info("Guest detected\n");
>>   }
> 
> AMD currently does:
> 
>          if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
>                  swiotlb_force = SWIOTLB_FORCE;
> 
> which somewhat begs the question of why we can't do the
> 
> 	swiotlb_force = SWIOTLB_FORCE;
> 
> thing in:
> 
> void __init mem_encrypt_init(void)
> {
>          if (!cc_platform_has(CC_ATTR_MEM_ENCRYPT))

If you make this cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT), then it 
should work for both, I would think. If you use CC_ATTR_MEM_ENCRYPT, 
you'll force bare-metal SME to always use bounce buffers when doing I/O. 
But SME can do I/O to encrypted memory if the device supports 64-bit DMA 
or if the IOMMU is being used, so we don't want to force SWIOTLB in this case.

Thanks,
Tom

>                  return;
> 
> /// Here
> 
> I recall there being a reason for this.  But I don't see any mention in
> the changelog.