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

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Date:   Wed, 9 Mar 2022 12:07:22 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "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,
        thomas.lendacky@....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 3/2/22 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))
                return;

/// Here

I recall there being a reason for this.  But I don't see any mention in
the changelog.