linux-kernel - Re: SEV guest regression in 4.18

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Date:   Tue, 21 Aug 2018 10:39:11 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>
Subject: Re: SEV guest regression in 4.18

On Mon, Aug 20, 2018 at 05:11:53PM -0500, Brijesh Singh wrote:
> Hi All,
> 
> The following commit
> 
> "
> x86/kvmclock: Remove memblock dependency
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=368a540e0232ad446931f5a4e8a5e06f69f21343
> 
> "
> broke the SEV support in 4.18.
> 
> Since the guest physical address holding the wall_clock and
> vcpu_time_info are shared with the hypervisor  it must be mapped
> as "decrypted" when SEV is active. To clear the C-bit we use
> kernel_physical_mapping_init() to split the large pages into 4K before
> changing the C-bit. Now the kernel_physical_mapping_init() is failing to
> allocate the memory because its called very early.
> 
> [    0.000000] Call Trace:
> [    0.000000]  ? dump_stack+0x5c/0x80
> [    0.000000]  ? panic+0xe7/0x247
> [    0.000000]  ? alloc_low_pages+0x130/0x130
> [    0.000000]  ? kernel_physical_mapping_init+0xe0/0x204
> [    0.000000]  ? early_set_memory_enc_dec+0x123/0x174
> [    0.000000]  ? 0xffffffffae000000
> [    0.000000]  ? kvmclock_init+0x60/0x1ea
> [    0.000000]  ? kvm_init_platform+0xa/0x16
> [    0.000000]  ? setup_arch+0x434/0xce9
> [    0.000000]  ? start_kernel+0x67/0x52e
> [    0.000000]  ? load_ucode_bsp+0x76/0x12e
> [    0.000000]  ? secondary_startup_64+0xa4/0xb0

Silly question: can we delay the call to kernel_physical_mapping_init()
to right before those variables are accessed by the guest for the first
time, assuming we can allocate memory at that point?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--