linux-kernel - Re: [PATCH RFC v8 24/56] crypto: ccp: Handle the legacy TMR allocation when SNP is enabled

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Message-ID: <f70a2398-bd78-24aa-b0ae-9171465d50ff@amd.com>
Date:   Tue, 21 Feb 2023 09:31:01 -0600
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     Zhi Wang <zhi.wang.linux@...il.com>,
        Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, 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, 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, nikunj.dadhania@....com,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH RFC v8 24/56] crypto: ccp: Handle the legacy TMR
 allocation when SNP is enabled

>> +static int snp_reclaim_pages(unsigned long paddr, unsigned int npages, bool locked)
>> +{
>> +	/* Cbit maybe set in the paddr */
> 
> This is confusing.
> 
> I suppose C-bit is treated as a attribute of PTE in the kernel not part of the
> PA. It means only a PTE might carry a C-bit.
> 

snp_reclaim_pages() is also called for reclaiming guest memory, in which 
case the (guest) paddr will have the C-bit set. Hence this C-bit 
handling is done within snp_reclaim_pages() so that the callers don't 
need to handle it explicitly.


> The paddr is from __pa(page_address()). It is not extracted from a PTE. Thus, the
> return from them should never have a C-bit.
> 
> BTW: Wouldn't it be better to have pfn as input param instead of paddr?
> 
> The caller has struct page, calling snp_reclaim_pages(page_to_pfn(page), xxxxx)
> would be much clearer than the current conversion:
> page_address() (struct page is converted to VA), __pa() (VA is converted to PA)
> in the caller and then PA is converted to pfn here.
> 
>> +	unsigned long pfn = __sme_clr(paddr) >> PAGE_SHIFT;
>> +	int ret, err, i, n = 0;
>> +
> 
> should be unsigned int i, n; as the input param npage is unsigned int.
> 
>> +	if (!pfn_valid(pfn)) {
>> +		pr_err("%s: Invalid PFN %lx\n", __func__, pfn);
>> +		return 0;
>> +	}
>> +
>> +	for (i = 0; i < npages; i++, pfn++, n++) {
>> +		paddr = pfn << PAGE_SHIFT;
>> +
>> +		if (locked)
>> +			ret = __sev_do_cmd_locked(SEV_CMD_SNP_PAGE_RECLAIM, &paddr, &err);
>> +		else
>> +			ret = sev_do_cmd(SEV_CMD_SNP_PAGE_RECLAIM, &paddr, &err);
>> +
>> +		if (ret)
>> +			goto cleanup;
>> +
>> +		ret = rmp_make_shared(pfn, PG_LEVEL_4K);
>> +		if (ret)
>> +			goto cleanup;
>> +	}
>> +
>> +	return 0;
>> +
>> +cleanup:
>> +	/*
>> +	 * If failed to reclaim the page then page is no longer safe to
>> +	 * be release back to the system, leak it.
>> +	 */
>> +	snp_mark_pages_offline(pfn, npages - n);
>> +	return ret;
>> +}
>> +
>> +static int rmp_mark_pages_firmware(unsigned long paddr, unsigned int npages, bool locked)
> 
> The same comment as above. Better take pfn or page instead of paddr with
> redundant conversions.
> 

Again, the paddr can point to guest memory so it can have C-bit set.

Thanks,
Ashish

>> +{
>> +	/* Cbit maybe set in the paddr */
>> +	unsigned long pfn = __sme_clr(paddr) >> PAGE_SHIFT;
>> +	int rc, n = 0, i;
>> +
>> +	for (i = 0; i < npages; i++, n++, pfn++) {
>> +		rc = rmp_make_private(pfn, 0, PG_LEVEL_4K, 0, true);
>> +		if (rc)
>> +			goto cleanup;
>> +	}
>> +
>> +	return 0;
>> +
>> +cleanup:
>> +	/*
>> +	 * Try unrolling the firmware state changes by
>> +	 * reclaiming the pages which were already changed to the
>> +	 * firmware state.
>> +	 */
>> +	snp_reclaim_pages(paddr, n, locked);
>> +
>> +	return rc;
>> +}
>> +