linux-kernel - Re: [PATCH v2 0/7] Provide support for RMPREAD and a segmented RMP

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Message-ID: <20240829065636.GBZtAbpOVh0lHr7VxG@fat_crate.local>
Date: Thu, 29 Aug 2024 08:56:36 +0200
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Michael Roth <michael.roth@....com>,
	Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v2 0/7] Provide support for RMPREAD and a segmented RMP

On Tue, Jul 30, 2024 at 02:40:00PM -0500, Tom Lendacky wrote:
> This series adds SEV-SNP support for a new instruction to read an RMP
> entry and for a segmented RMP table.
> 
> The RMPREAD instruction is used to return information related to an RMP
> entry in an architecturally defined format.
> 
> RMPREAD support is detected via CPUID 0x8000001f_EAX[21].
> 
> Segmented RMP support is a new way of representing the layout of an RMP
> table. Initial RMP table support required the RMP table to be contiguous
> in memory. RMP accesses from a NUMA node on which the RMP doesn't reside
> can take longer than accesses from a NUMA node on which the RMP resides.
> Segmented RMP support allows the RMP entries to be located on the same
> node as the memory the RMP is covering, potentially reducing latency
> associated with accessing an RMP entry associated with the memory. Each
> RMP segment covers a specific range of system physical addresses.
> 
> Segmented RMP support is detected and established via CPUID and MSRs.
> 
> CPUID:
>   - 0x8000001f_EAX[23]
>     - Indicates support for segmented RMP
> 
>   - 0x80000025_EAX
>     - [5:0]   : Minimum supported RMP segment size
>     - [11:6]  : Maximum supported RMP segment size
> 
>   - 0x80000025_EBX
>     - [9:0]   : Number of cacheable RMP segment definitions
>     - [10]    : Indicates if the number of cacheable RMP segments is
>                 a hard limit
> 
> MSR:
>   - 0xc0010132 (RMP_BASE)
>     - Is identical to current RMP support
> 
>   - 0xc0010133 (RMP_END)
>     - Should be in reset state if segment RMP support is active
> 
>       For kernels that do not support segmented RMP, being in reset
>       state allows the kernel to disable SNP support if the non-segmented
>       RMP has not been allocated.
> 
>   - 0xc0010136 (RMP_CFG)
>     - [0]    : Indicates if segmented RMP is enabled
>     - [13:8] : Contains the size of memory covered by an RMP segment
>                (expressed as a power of 2)
> 
> The RMP segment size defined in the RMP_CFG MSR applies to all segments
> of the RMP. Therefore each RMP segment covers a specific range of system
> physical addresses. For example, if the RMP_CFG MSR value is 0x2401, then
> the RMP segment coverage value is 0x24 => 36, meaning the size of memory
> covered by an RMP segment is 64GB (1 << 36). So the first RMP segment
> covers physical addresses from 0 to 0xF_FFFF_FFFF, the second RMP segment
> covers physical addresses from 0x10_0000_0000 to 0x1F_FFFF_FFFF, etc.
> 
> When a segmented RMP is enabled, RMP_BASE points to the RMP bookkeeping
> area as it does today (16K in size). However, instead of RMP entries
> beginning immediately after the bookkeeping area, there is a 4K RMP
> segment table. Each entry in the table is 8-bytes in size:
> 
>   - [19:0]  : Mapped size (in GB)
>               The mapped size can be less than the defined segment size.
>               A value of zero, indicates that no RMP exists for the range
>               of system physical addresses associated with this segment.
>     [51:20] : Segment physical address
>               This address is left shift 20-bits (or just masked when
>               read) to form the physical address of the segment (1MB
>               alignment).

I could very well use that nicely gathered together info on segmented RMP if
it were, say, in a section in

Documentation/arch/x86/amd-memory-encryption.rst

for example.

:-)

Thx.

-- 
Regards/Gruss,
    Boris.

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