linux-kernel - Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption status is changed

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Date:   Wed, 12 May 2021 15:15:37 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ashish Kalra <Ashish.Kalra@....com>, seanjc@...gle.com
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, joro@...tes.org, thomas.lendacky@....com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        srutherford@...gle.com, venu.busireddy@...cle.com,
        brijesh.singh@....com
Subject: Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption
 status is changed

On Fri, Apr 23, 2021 at 03:58:43PM +0000, Ashish Kalra wrote:
> +static inline void notify_page_enc_status_changed(unsigned long pfn,
> +						  int npages, bool enc)
> +{
> +	PVOP_VCALL3(mmu.notify_page_enc_status_changed, pfn, npages, enc);
> +}

Now the question is whether something like that is needed for TDX, and,
if so, could it be shared by both.

Sean?

> +void notify_addr_enc_status_changed(unsigned long vaddr, int npages,
> +				    bool enc)

Let that line stick out.

> +{
> +#ifdef CONFIG_PARAVIRT
> +	unsigned long sz = npages << PAGE_SHIFT;
> +	unsigned long vaddr_end = vaddr + sz;
> +
> +	while (vaddr < vaddr_end) {
> +		int psize, pmask, level;
> +		unsigned long pfn;
> +		pte_t *kpte;
> +
> +		kpte = lookup_address(vaddr, &level);
> +		if (!kpte || pte_none(*kpte))
> +			return;

What does this mean exactly? On the first failure to lookup the address,
you return? Why not continue so that you can notify about the remaining
pages in [vaddr - vaddr_end)?

Also, what does it mean for the current range if the lookup fails?
Innocuous situation or do you need to signal it with a WARN or so?

> +
> +		pfn = pg_level_to_pfn(level, kpte, NULL);
> +		if (!pfn)
> +			continue;

Same here: if it hits the default case, wouldn't it make sense to
WARN_ONCE or so to catch potential misuse? Or better yet, the WARN_ONCE
should be in pg_level_to_pfn().

> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 16f878c26667..45e65517405a 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -2012,6 +2012,13 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
>  	 */
>  	cpa_flush(&cpa, 0);
>  
> +	/*
> +	 * Notify hypervisor that a given memory range is mapped encrypted
> +	 * or decrypted. The hypervisor will use this information during the
> +	 * VM migration.
> +	 */
> +	notify_addr_enc_status_changed(addr, numpages, enc);

If you notify about a range then that function should be called

	notify_range_enc_status_changed

or so.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette