linux-kernel - Re: [PATCHv3 09/14] x86/tdx: Account shared memory

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Date:   Tue, 21 Nov 2023 02:47:29 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "ashish.kalra@....com" <ashish.kalra@....com>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "bhe@...hat.com" <bhe@...hat.com>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>
Subject: Re: [PATCHv3 09/14] x86/tdx: Account shared memory


> +static atomic_long_t nr_shared;
> +
> +static inline bool pte_decrypted(pte_t pte)
> +{
> +	return cc_mkdec(pte_val(pte)) == pte_val(pte);
> +}
> +
>  /* Called from __tdx_hypercall() for unrecoverable failure */
>  noinstr void __noreturn __tdx_hypercall_failed(void)
>  {
> @@ -820,6 +828,11 @@ static int tdx_enc_status_change_finish(unsigned long vaddr, int numpages,
>  	if (!enc && !tdx_enc_status_changed(vaddr, numpages, enc))
>  		return -EIO;
>  
> +	if (enc)
> +		atomic_long_sub(numpages, &nr_shared);
> +	else
> +		atomic_long_add(numpages, &nr_shared);
> +
>  	return 0;
>  }
>  
> @@ -895,3 +908,59 @@ void __init tdx_early_init(void)
>  
>  	pr_info("Guest detected\n");
>  }
> +
> +#ifdef CONFIG_DEBUG_FS
> +static int tdx_shared_memory_show(struct seq_file *m, void *p)
> +{
> +	unsigned long addr, end;
> +	unsigned long found = 0;
> +
> +	addr = PAGE_OFFSET;
> +	end  = PAGE_OFFSET + get_max_mapped();
> +
> +	while (addr < end) {
> +		unsigned long size;
> +		unsigned int level;
> +		pte_t *pte;
> +
> +		pte = lookup_address(addr, &level);
> +		size = page_level_size(level);
> +
> +		if (pte && pte_decrypted(*pte))
> +			found += size / PAGE_SIZE;
> +
> +		addr += size;
> +
> +		cond_resched();
> +	}
> +
> +	seq_printf(m, "Number of unshared pages in kernel page tables:  %16lu\n",
> +		   found);
> +	seq_printf(m, "Number of pages accounted as unshared:           %16ld\n",
> +		   atomic_long_read(&nr_shared));

unshared -> shared?

Btw, I am not quite sure what's the purpose of reporting number of shared pages
in both kernel page table and that the kernel is accounting?

IIUC, there might be slight chance that the former is different from the latter
(i.e., when user reads this while the kernel is converting pages
simultaneously), but in most of the time the user should see they are the same.

I can see it might be helpful to report @nr_shared to the user, but how can
reporting both help the user?

That being said, I think perhaps you can separate the /sysfs part as a separate
patch because it's not a mandatory part of this series but a nice to have.  Then
the /sysfs part can be reviewed separately.