linux-kernel - Re: [RFC PATCH v2 12/20] x86: Add support for changing memory encryption attribute

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Date:   Fri, 9 Sep 2016 19:23:15 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     linux-arch@...r.kernel.org, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Joerg Roedel <joro@...tes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [RFC PATCH v2 12/20] x86: Add support for changing memory
 encryption attribute

On Mon, Aug 22, 2016 at 05:37:49PM -0500, Tom Lendacky wrote:
> This patch adds support to be change the memory encryption attribute for
> one or more memory pages.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/include/asm/cacheflush.h  |    3 +
>  arch/x86/include/asm/mem_encrypt.h |   13 ++++++
>  arch/x86/mm/mem_encrypt.c          |   43 +++++++++++++++++++++
>  arch/x86/mm/pageattr.c             |   75 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 134 insertions(+)

...

> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 72c292d..0ba9382 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -1728,6 +1728,81 @@ int set_memory_4k(unsigned long addr, int numpages)
>  					__pgprot(0), 1, 0, NULL);
>  }
>  
> +static int __set_memory_enc_dec(struct cpa_data *cpa)
> +{
> +	unsigned long addr;
> +	int numpages;
> +	int ret;
> +
> +	if (*cpa->vaddr & ~PAGE_MASK) {
> +		*cpa->vaddr &= PAGE_MASK;
> +
> +		/* People should not be passing in unaligned addresses */
> +		WARN_ON_ONCE(1);

Let's make this more user-friendly:

	if (WARN_ONCE(*cpa->vaddr & ~PAGE_MASK, "Misaligned address: 0x%lx\n", *cpa->vaddr))
		*cpa->vaddr &= PAGE_MASK;

> +	}
> +
> +	addr = *cpa->vaddr;
> +	numpages = cpa->numpages;
> +
> +	/* Must avoid aliasing mappings in the highmem code */
> +	kmap_flush_unused();
> +	vm_unmap_aliases();
> +
> +	ret = __change_page_attr_set_clr(cpa, 1);
> +
> +	/* Check whether we really changed something */
> +	if (!(cpa->flags & CPA_FLUSHTLB))
> +		goto out;
> +
> +	/*
> +	 * On success we use CLFLUSH, when the CPU supports it to
> +	 * avoid the WBINVD.
> +	 */
> +	if (!ret && static_cpu_has(X86_FEATURE_CLFLUSH))
> +		cpa_flush_range(addr, numpages, 1);
> +	else
> +		cpa_flush_all(1);

So if we fail (ret != 0) we do WBINVD unconditionally even if we don't
have to?

Don't you want this instead:

        ret = __change_page_attr_set_clr(cpa, 1);
        if (ret)
                goto out;

        /* Check whether we really changed something */
        if (!(cpa->flags & CPA_FLUSHTLB))
                goto out;

        /*
         * On success we use CLFLUSH, when the CPU supports it to
         * avoid the WBINVD.
         */
        if (static_cpu_has(X86_FEATURE_CLFLUSH))
                cpa_flush_range(addr, numpages, 1);
        else
                cpa_flush_all(1);

out:
        return ret;
}

?

-- 
Regards/Gruss,
    Boris.

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