linux-kernel - Re: [Patch v3 07/14] x86/hyperv: Change vTOM handling to use standard coco mechanisms

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Date:   Mon, 21 Nov 2022 16:03:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     hpa@...or.com, kys@...rosoft.com, haiyangz@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, luto@...nel.org,
        peterz@...radead.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, lpieralisi@...nel.org,
        robh@...nel.org, kw@...ux.com, bhelgaas@...gle.com, arnd@...db.de,
        hch@...radead.org, m.szyprowski@...sung.com, robin.murphy@....com,
        thomas.lendacky@....com, brijesh.singh@....com, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com,
        Tianyu.Lan@...rosoft.com, kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, dan.j.williams@...el.com,
        jane.chu@...cle.com, seanjc@...gle.com, tony.luck@...el.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-arch@...r.kernel.org,
        iommu@...ts.linux.dev
Subject: Re: [Patch v3 07/14] x86/hyperv: Change vTOM handling to use
 standard coco mechanisms

On Wed, Nov 16, 2022 at 10:41:30AM -0800, Michael Kelley wrote:
> Hyper-V guests on AMD SEV-SNP hardware have the option of using the
> "virtual Top Of Memory" (vTOM) feature specified by the SEV-SNP
> architecture. With vTOM, shared vs. private memory accesses are
> controlled by splitting the guest physical address space into two
> halves.  vTOM is the dividing line where the uppermost bit of the
> physical address space is set; e.g., with 47 bits of guest physical
> address space, vTOM is 0x40000000000 (bit 46 is set).  Guest phyiscal

Unknown word [phyiscal] in commit message.
Suggestions: ['physical', 'physically', ... ]

Please introduce a spellchecker into your patch creation workflow.

...

> @@ -108,6 +115,7 @@ u64 cc_mkenc(u64 val)
>  	switch (vendor) {
>  	case CC_VENDOR_AMD:
>  		return val | cc_mask;
> +	case CC_VENDOR_HYPERV:
>  	case CC_VENDOR_INTEL:
>  		return val & ~cc_mask;
>  	default:
> @@ -121,6 +129,7 @@ u64 cc_mkdec(u64 val)
>  	switch (vendor) {
>  	case CC_VENDOR_AMD:
>  		return val & ~cc_mask;
> +	case CC_VENDOR_HYPERV:
>  	case CC_VENDOR_INTEL:
>  		return val | cc_mask;
>  	default:

Uuuh, this needs a BIG FAT COMMENT.

You're running on SNP and yet the enc/dec meaning is flipped. And that's
because of vTOM.

What happens if you have other types of SNP-based VMs on HyperV? The
isolation VMs thing? Or is that the same?

What happens when you do TDX guests with HyperV?

This becomes wrong then.

I think you need a more finer-grained check here in the sense of "is it
a HyperV guest using a paravisor and vTOM is enabled" or so.

Otherwise, I like the removal of the HyperV-specific checks ofc.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette