linux-kernel - Re: [PATCH v3 1/8] x86/sev: Prepare for using the RMPREAD instruction to access the RMP

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Message-ID: <85583be0-5e8a-82ce-a134-95fe9d85a2a7@amd.com>
Date: Wed, 16 Oct 2024 14:22:22 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Tom Lendacky <thomas.lendacky@....com>, linux-kernel@...r.kernel.org,
 x86@...nel.org
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Michael Roth <michael.roth@....com>, Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v3 1/8] x86/sev: Prepare for using the RMPREAD instruction
 to access the RMP

On 9/30/2024 8:52 PM, Tom Lendacky wrote:
> The RMPREAD instruction returns an architecture defined format of an
> RMP entry. This is the preferred method for examining RMP entries.
> 
> In preparation for using the RMPREAD instruction, convert the existing
> code that directly accesses the RMP to map the raw RMP information into
> the architecture defined format.
> 
> RMPREAD output returns a status bit for the 2MB region status. If the
> input page address is 2MB aligned and any other pages within the 2MB
> region are assigned, then 2MB region status will be set to 1. Otherwise,
> the 2MB region status will be set to 0. For systems that do not support
> RMPREAD, calculating this value would require looping over all of the RMP
> table entries within that range until one is found with the assigned bit
> set. Since this bit is not defined in the current format, and so not used
> today, do not incur the overhead associated with calculating it.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/virt/svm/sev.c | 141 ++++++++++++++++++++++++++++------------
>  1 file changed, 98 insertions(+), 43 deletions(-)
> 
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index 0ce17766c0e5..103a2dd6e81d 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -30,11 +30,27 @@
>  #include <asm/cmdline.h>
>  #include <asm/iommu.h>
>  
> +/*
> + * The RMP entry format as returned by the RMPREAD instruction.
> + */
> +struct rmpentry {
> +	u64 gpa;
> +	u8  assigned		:1,
> +	    rsvd1		:7;
> +	u8  pagesize		:1,
> +	    hpage_region_status	:1,
> +	    rsvd2		:6;
> +	u8  immutable		:1,
> +	    rsvd3		:7;
> +	u8  rsvd4;
> +	u32 asid;
> +} __packed;
> +
>  /*
>   * The RMP entry format is not architectural. The format is defined in PPR
>   * Family 19h Model 01h, Rev B1 processor.
>   */
> -struct rmpentry {
> +struct rmpentry_raw {
>  	union {
>  		struct {
>  			u64 assigned	: 1,
> @@ -62,7 +78,7 @@ struct rmpentry {
>  #define PFN_PMD_MASK	GENMASK_ULL(63, PMD_SHIFT - PAGE_SHIFT)
>  
>  static u64 probed_rmp_base, probed_rmp_size;
> -static struct rmpentry *rmptable __ro_after_init;
> +static struct rmpentry_raw *rmptable __ro_after_init;
>  static u64 rmptable_max_pfn __ro_after_init;
>  
>  static LIST_HEAD(snp_leaked_pages_list);
> @@ -247,8 +263,8 @@ static int __init snp_rmptable_init(void)
>  	rmptable_start += RMPTABLE_CPU_BOOKKEEPING_SZ;
>  	rmptable_size = probed_rmp_size - RMPTABLE_CPU_BOOKKEEPING_SZ;
>  
> -	rmptable = (struct rmpentry *)rmptable_start;
> -	rmptable_max_pfn = rmptable_size / sizeof(struct rmpentry) - 1;
> +	rmptable = (struct rmpentry_raw *)rmptable_start;
> +	rmptable_max_pfn = rmptable_size / sizeof(struct rmpentry_raw) - 1;
>  
>  	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "x86/rmptable_init:online", __snp_enable, NULL);
>  
> @@ -270,48 +286,77 @@ static int __init snp_rmptable_init(void)
>   */
>  device_initcall(snp_rmptable_init);
>  
> -static struct rmpentry *get_rmpentry(u64 pfn)
> +static struct rmpentry_raw *__get_rmpentry(unsigned long pfn)

pfn type has changed from u64 => unsigned long, is this intentional ?

>  {
> -	if (WARN_ON_ONCE(pfn > rmptable_max_pfn))
> -		return ERR_PTR(-EFAULT);
> -
> -	return &rmptable[pfn];
> -}
> -
> -static struct rmpentry *__snp_lookup_rmpentry(u64 pfn, int *level)
> -{
> -	struct rmpentry *large_entry, *entry;
> -
> -	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
> +	if (!rmptable)
>  		return ERR_PTR(-ENODEV);
>  
> -	entry = get_rmpentry(pfn);
> -	if (IS_ERR(entry))
> -		return entry;
> +	if (unlikely(pfn > rmptable_max_pfn))
> +		return ERR_PTR(-EFAULT);
> +
> +	return rmptable + pfn;
> +}
> +
> +static int get_rmpentry(u64 pfn, struct rmpentry *entry)
> +{
> +	struct rmpentry_raw *e;
> +
> +	e = __get_rmpentry(pfn);
> +	if (IS_ERR(e))
> +		return PTR_ERR(e);
> +
> +	/*
> +	 * Map the RMP table entry onto the RMPREAD output format.
> +	 * The 2MB region status indicator (hpage_region_status field) is not
> +	 * calculated, since the overhead could be significant and the field
> +	 * is not used.
> +	 */
> +	memset(entry, 0, sizeof(*entry));
> +	entry->gpa       = e->gpa << PAGE_SHIFT;
> +	entry->asid      = e->asid;
> +	entry->assigned  = e->assigned;
> +	entry->pagesize  = e->pagesize;
> +	entry->immutable = e->immutable;
> +
> +	return 0;
> +}
> +
> +static int __snp_lookup_rmpentry(u64 pfn, struct rmpentry *entry, int *level)
> +{
> +	struct rmpentry large_entry;
> +	int ret;
> +
> +	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
> +		return -ENODEV;

Can we rely on rmp_table check in __get_rmpentry() and remove the above check ?
If rmp_table is NULL, CC_ATTR_HOST_SEV_SNP is always cleared.

> +
> +	ret = get_rmpentry(pfn, entry);
> +	if (ret)
> +		return ret;

Regards
Nikunj