linux-kernel - Re: [PATCH RFC v7 16/64] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction

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Date:   Wed, 8 Feb 2023 16:30:16 +0000
From:   Liam Merwick <liam.merwick@...cle.com>
To:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc:     linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        luto@...nel.org, dave.hansen@...ux.intel.com, slp@...hat.com,
        pgonda@...gle.com, peterz@...radead.org,
        srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com,
        dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de,
        vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        harald@...fian.com, Brijesh Singh <brijesh.singh@....com>,
        Liam Merwick <liam.merwick@...cle.com>
Subject: Re: [PATCH RFC v7 16/64] x86/sev: Add helper functions for RMPUPDATE
 and PSMASH instruction

On 14/12/2022 19:40, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@....com>
> 
> The RMPUPDATE instruction writes a new RMP entry in the RMP Table. The
> hypervisor will use the instruction to add pages to the RMP table. See
> APM3 for details on the instruction operations.
> 
> The PSMASH instruction expands a 2MB RMP entry into a corresponding set
> of contiguous 4KB-Page RMP entries. The hypervisor will use this
> instruction to adjust the RMP entry without invalidating the previous
> RMP entry.
> 
> Add the following external interface API functions:
> 
> int psmash(u64 pfn);
> psmash is used to smash a 2MB aligned page into 4K
> pages while preserving the Validated bit in the RMP.
> 
> int rmp_make_private(u64 pfn, u64 gpa, enum pg_level level, int asid, bool immutable);
> Used to assign a page to guest using the RMPUPDATE instruction.
> 
> int rmp_make_shared(u64 pfn, enum pg_level level);
> Used to transition a page to hypervisor/shared state using the RMPUPDATE instruction.
> 
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> [mdr: add RMPUPDATE retry logic for transient FAIL_OVERLAP errors]
> Signed-off-by: Michael Roth <michael.roth@....com>
> ---
>   arch/x86/include/asm/sev.h | 24 ++++++++++
>   arch/x86/kernel/sev.c      | 95 ++++++++++++++++++++++++++++++++++++++
>   2 files changed, 119 insertions(+)
> 

...

> +
> +static int rmpupdate(u64 pfn, struct rmp_state *val)
> +{
> +	unsigned long paddr = pfn << PAGE_SHIFT;
> +	int retries = 0;
> +	int ret;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return -ENXIO;
> +
> +retry:
> +	/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> +	asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
> +		     : "=a"(ret)
> +		     : "a"(paddr), "c"((unsigned long)val)
> +		     : "memory", "cc");
> +
> +	if (ret) {
> +		if (!retries) {
> +			pr_err("RMPUPDATE failed, ret: %d, pfn: %llx, npages: %d, level: %d, retrying (max: %d)...\n",
> +			       ret, pfn, npages, level, 2 * num_present_cpus());

This patch isn't bisectable - 'npages' isn't defined in this patch - 
it's defined later in Patch18

otherwise LGTM

Regards,
Liam


> +			dump_stack();
> +		}
> +		retries++;
> +		if (retries < 2 * num_present_cpus())
> +			goto retry;
> +	} else if (retries > 0) {
> +		pr_err("RMPUPDATE for pfn %llx succeeded after %d retries\n", pfn, retries);
> +	}
> +
> +	return ret;
> +}