Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table

[Sean Christopherson]

On Wed, Jul 07, 2021, Brijesh Singh wrote:
> The integrity guarantee of SEV-SNP is enforced through the RMP table.
> The RMP is used in conjuntion with standard x86 and IOMMU page
> tables to enforce memory restrictions and page access rights. The
> RMP is indexed by system physical address, and is checked at the end
> of CPU and IOMMU table walks. The RMP check is enforced as soon as
> SEV-SNP is enabled globally in the system. Not every memory access
> requires an RMP check. In particular, the read accesses from the
> hypervisor do not require RMP checks because the data confidentiality
> is already protected via memory encryption. When hardware encounters
> an RMP checks failure, it raise a page-fault exception. The RMP bit in
> fault error code can be used to determine if the fault was due to an
> RMP checks failure.
> 
> A write from the hypervisor goes through the RMP checks. When the
> hypervisor writes to pages, hardware checks to ensures that the assigned
> bit in the RMP is zero (i.e page is shared). If the page table entry that
> gives the sPA indicates that the target page size is a large page, then
> all RMP entries for the 4KB constituting pages of the target must have the
> assigned bit 0. If one of entry does not have assigned bit 0 then hardware
> will raise an RMP violation. To resolve it, split the page table entry
> leading to target page into 4K.

Isn't the above just saying:

  All RMP entries covered by a large page must match the shared vs. encrypted
  state of the page, e.g. host large pages must have assigned=0 for all relevant
  RMP entries.

> This poses a challenge in the Linux memory model. The Linux kernel
> creates a direct mapping of all the physical memory -- referred to as
> the physmap. The physmap may contain a valid mapping of guest owned pages.
> During the page table walk, the host access may get into the situation
> where one of the pages within the large page is owned by the guest (i.e
> assigned bit is set in RMP). A write to a non-guest within the large page
> will raise an RMP violation. Call set_memory_4k() to split the physmap
> before adding the page in the RMP table. This ensures that the pages
> added in the RMP table are used as 4K in the physmap.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/kernel/sev.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 949efe530319..a482e01f880a 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2375,6 +2375,12 @@ int rmpupdate(struct page *page, struct rmpupdate *val)
>  	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
>  		return -ENXIO;
>  
> +	ret = set_memory_4k((unsigned long)page_to_virt(page), 1);

IIUC, this shatters the direct map for page that's assigned to an SNP guest, and
the large pages are never recovered?

I believe a better approach would be to do something similar to memfd_secret[*],
which encountered a similar problem with the direct map.  Instead of forcing the
direct map to be forever 4k, unmap the direct map when making a page guest private,
and restore the direct map when it's made shared (or freed).

I thought memfd_secret had also solved the problem of restoring large pages in
the direct map, but at a glance I can't tell if that's actually implemented
anywhere.  But, even if it's not currently implemented, I think it makes sense
to mimic the memfd_secret approach so that both features can benefit if large
page preservation/restoration is ever added.

[*] https://lkml.kernel.org/r/20210518072034.31572-5-rppt@xxxxxxxxxx

> +	if (ret) {
> +		pr_err("Failed to split physical address 0x%lx (%d)\n", spa, ret);
> +		return ret;
> +	}
> +
>  	/* Retry if another processor is modifying the RMP entry. */
>  	do {
>  		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> -- 
> 2.17.1
>