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

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Date:   Tue, 21 Feb 2023 16:06:06 -0600
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     Zhi Wang <zhi.wang.linux@...il.com>
Cc:     Michael Roth <michael.roth@....com>, 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

On 2/21/2023 3:15 PM, Zhi Wang wrote:
> On Tue, 21 Feb 2023 09:31:01 -0600
> "Kalra, Ashish" <ashish.kalra@....com> wrote:
> 
>>>> +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.
> 
> Thanks for the explanation.
> 
> Do you mean it will be used like that in the later patch? Sorry if it is in the
> later patch as I was making progress slowly. It is quite a big patch set.
>

Yes, these are callers in later patches, like the following code path in 
patch 25:

static int unmap_firmware_writeable(u64 *paddr, u32 len, bool guest, 
struct snp_host_map *map)
{
         unsigned int npages = PAGE_ALIGN(len) >> PAGE_SHIFT;
	...
         /* If paddr points to a guest memory then restore the page 
state to hypervisor. */
         if (guest) {
                 if (snp_reclaim_pages(*paddr, npages, true))
                         return -EFAULT;

                 goto done;
         }

       	...
	...

Or, the following as part of patch 52:

int snp_guest_dbg_decrypt_page(u64 gctx_pfn, u64 src_pfn, u64 dst_pfn, 
int *error)
{
	...
         data.gctx_paddr = sme_me_mask | (gctx_pfn << PAGE_SHIFT);
         data.src_addr = sme_me_mask | (src_pfn << PAGE_SHIFT);
         data.dst_addr = sme_me_mask | (dst_pfn << PAGE_SHIFT);

         /* The destination page must be in the firmware state. */
         if (rmp_mark_pages_firmware(data.dst_addr, 1, false))
                 return -EIO;

         ret = sev_do_cmd(SEV_CMD_SNP_DBG_DECRYPT, &data, error);

         /* Restore the page state */
         if (snp_reclaim_pages(data.dst_addr, 1, false))
	...
	...

Thanks,
Ashish

> At least, I don't see that kind of usage in the current patch. Feel free to
> correct me if I am wrong.
> 
> The call chains:
> 
> __snp_free_firmware_page()
>      snp_reclaim_pages();
> 
> As __snp_free_firmware_page() takes struct page*, all the follwing coversion
> from it would not carry C-bit.
> 
> __snp_alloc_firmware_pages()
>    rmp_mark_pages_firmware()
>      snp_reclaim_pages()
> 
> As __snp_alloc_firmware_page() allocates page with struct page*, the same
> conclusion as above.
> 
>>
>>
>>> 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;
>>>> +}
>>>> +
>