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Date:   Thu, 17 Nov 2022 10:59:32 +0800
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Subject: Re: [Patch v3 07/14] x86/hyperv: Change vTOM handling to use standard
 coco mechanisms
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To:     Michael Kelley <mikelley@microsoft.com>, hpa@zytor.com,
        kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org,
        decui@microsoft.com, luto@kernel.org, peterz@infradead.org,
        davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, lpieralisi@kernel.org, robh@kernel.org,
        kw@linux.com, bhelgaas@google.com, arnd@arndb.de,
        hch@infradead.org, m.szyprowski@samsung.com, robin.murphy@arm.com,
        thomas.lendacky@amd.com, brijesh.singh@amd.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        Tianyu.Lan@microsoft.com, kirill.shutemov@linux.intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com, ak@linux.intel.com,
        isaku.yamahata@intel.com, dan.j.williams@intel.com,
        jane.chu@oracle.com, seanjc@google.com, tony.luck@intel.com,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-hyperv@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, linux-arch@vger.kernel.org,
        iommu@lists.linux.dev
References: <1668624097-14884-1-git-send-email-mikelley@microsoft.com>
 <1668624097-14884-8-git-send-email-mikelley@microsoft.com>
From:   Tianyu Lan <ltykernel@gmail.com>
In-Reply-To: <1668624097-14884-8-git-send-email-mikelley@microsoft.com>
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On 11/17/2022 2:41 AM, 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
> memory is accessible at two parallel physical addresses -- one below
> vTOM and one above vTOM.  Accesses below vTOM are private (encrypted)
> while accesses above vTOM are shared (decrypted). In this sense, vTOM
> is like the GPA.SHARED bit in Intel TDX.
> 
> Support for Hyper-V guests using vTOM was added to the Linux kernel in
> two patch sets[1][2]. This support treats the vTOM bit as part of
> the physical address. For accessing shared (decrypted) memory, these
> patch sets create a second kernel virtual mapping that maps to physical
> addresses above vTOM.
> 
> A better approach is to treat the vTOM bit as a protection flag, not
> as part of the physical address. This new approach is like the approach
> for the GPA.SHARED bit in Intel TDX. Rather than creating a second kernel
> virtual mapping, the existing mapping is updated using recently added
> coco mechanisms.  When memory is changed between private and shared using
> set_memory_decrypted() and set_memory_encrypted(), the PTEs for the
> existing kernel mapping are changed to add or remove the vTOM bit
> in the guest physical address, just as with TDX. The hypercalls to
> change the memory status on the host side are made using the existing
> callback mechanism. Everything just works, with a minor tweak to map
> the I/O APIC to use private accesses.
> 
> To accomplish the switch in approach, the following must be done in
> in this single patch:
> 
> * Update Hyper-V initialization to set the cc_mask based on vTOM
>    and do other coco initialization.
> 
> * Update physical_mask so the vTOM bit is no longer treated as part
>    of the physical address
> 
> * Update cc_mkenc() and cc_mkdec() to be active for Hyper-V guests.
>    This makes the vTOM bit part of the protection flags.
> 
> * Code already exists to make hypercalls to inform Hyper-V about pages
>    changing between shared and private.  Update this code to run as a
>    callback from __set_memory_enc_pgtable().
> 
> * Remove the Hyper-V special case from __set_memory_enc_dec()
> 
> * Remove the Hyper-V specific call to swiotlb_update_mem_attributes()
>    since mem_encrypt_init() will now do it.
> 
> [1]https://lore.kernel.org/all/20211025122116.264793-1-ltykernel@gmail.com/
> [2]https://lore.kernel.org/all/20211213071407.314309-1-ltykernel@gmail.com/
> 
> Signed-off-by: Michael Kelley<mikelley@microsoft.com>
> ---
>   arch/x86/coco/core.c            | 11 +++++++++-
>   arch/x86/hyperv/hv_init.c       | 11 ----------
>   arch/x86/hyperv/ivm.c           | 45 +++++++++++++++++++++++++++++++----------
>   arch/x86/include/asm/mshyperv.h |  8 ++------
>   arch/x86/kernel/cpu/mshyperv.c  | 15 +++++++-------
>   arch/x86/mm/pat/set_memory.c    |  3 ---
>   6 files changed, 53 insertions(+), 40 deletions(-)

Reviewed-by: Tianyu Lan <Tianyu.Lan@microsoft.com>