linux-kernel - Re: [RFC PATCH v4 26/28] x86: Allow kexec to be used with SME

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Date:   Mon, 6 Mar 2017 11:58:40 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Young <dyoung@...hat.com>
CC:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        <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>, Rik van Riel <riel@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Larry Woodman <lwoodman@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [RFC PATCH v4 26/28] x86: Allow kexec to be used with SME

On 3/1/2017 3:25 AM, Dave Young wrote:
> Hi Tom,

Hi Dave,

>
> On 02/17/17 at 10:43am, Tom Lendacky wrote:
>> On 2/17/2017 9:57 AM, Konrad Rzeszutek Wilk wrote:
>>> On Thu, Feb 16, 2017 at 09:47:55AM -0600, Tom Lendacky wrote:
>>>> Provide support so that kexec can be used to boot a kernel when SME is
>>>> enabled.
>>>
>>> Is the point of kexec and kdump to ehh, dump memory ? But if the
>>> rest of the memory is encrypted you won't get much, will you?
>>
>> Kexec can be used to reboot a system without going back through BIOS.
>> So you can use kexec without using kdump.
>>
>> For kdump, just taking a quick look, the option to enable memory
>> encryption can be provided on the crash kernel command line and then
>
> Is there a simple way to get the SME status? Probably add some sysfs
> file for this purpose.

Currently there is not.  I can look at adding something, maybe just the
sme_me_mask value, which if non-zero, would indicate SME is active.

>
>> crash kernel can would be able to copy the memory decrypted if the
>> pagetable is set up properly. It looks like currently ioremap_cache()
>> is used to map the old memory page.  That might be able to be changed
>> to a memremap() so that the encryption bit is set in the mapping. That
>> will mean that memory that is not marked encrypted (EFI tables, swiotlb
>> memory, etc) would not be read correctly.
>
> Manage to store info about those ranges which are not encrypted so that
> memremap can handle them?

I can look into whether something can be done in this area. Any input
you can provide as to what would be the best way/place to store the
range info so kdump can make use of it, would be greatly appreciated.

>
>>
>>>
>>> Would it make sense to include some printk to the user if they
>>> are setting up kdump that they won't get anything out of it?
>>
>> Probably a good idea to add something like that.
>
> It will break kdump functionality, it should be fixed instead of
> just adding printk to warn user..

I do want kdump to work. I'll investigate further what can be done in
this area.

Thanks,
Tom

>
> Thanks
> Dave
>