Linux Virtualization - Apr 2020 - [PATCH 40/70] x86/sev-es: Setup per-cpu GHCBs for the runtime handler

On 4/14/20 2:03 PM, Mike Stunes wrote:
> On Mar 19, 2020, at 2:13 AM, Joerg Roedel <joro at 8bytes.org> wrote:
>>
>> From: Tom Lendacky <thomas.lendacky at amd.com>
>>
>> The runtime handler needs a GHCB per CPU. Set them up and map them
>> unencrypted.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky at amd.com>
>> Signed-off-by: Joerg Roedel <jroedel at suse.de>
>> ---
>> arch/x86/include/asm/mem_encrypt.h |  2 ++
>> arch/x86/kernel/sev-es.c           | 28 +++++++++++++++++++++++++++-
>> arch/x86/kernel/traps.c            |  3 +++
>> 3 files changed, 32 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
>> index c17980e8db78..4bf5286310a0 100644
>> --- a/arch/x86/kernel/sev-es.c
>> +++ b/arch/x86/kernel/sev-es.c
>> @@ -197,6 +203,26 @@ static bool __init sev_es_setup_ghcb(void)
>> 	return true;
>> }
>>
>> +void sev_es_init_ghcbs(void)
>> +{
>> +	int cpu;
>> +
>> +	if (!sev_es_active())
>> +		return;
>> +
>> +	/* Allocate GHCB pages */
>> +	ghcb_page = __alloc_percpu(sizeof(struct ghcb), PAGE_SIZE);
>> +
>> +	/* Initialize per-cpu GHCB pages */
>> +	for_each_possible_cpu(cpu) {
>> +		struct ghcb *ghcb = (struct ghcb *)per_cpu_ptr(ghcb_page, cpu);
>> +
>> +		set_memory_decrypted((unsigned long)ghcb,
>> +				     sizeof(*ghcb) >> PAGE_SHIFT);
>> +		memset(ghcb, 0, sizeof(*ghcb));
>> +	}
>> +}
>> +
> 
> set_memory_decrypted needs to check the return value. I see it
> consistently return ENOMEM. I've traced that back to split_large_page
> in arch/x86/mm/pat/set_memory.c.
At that point the guest won't be able to communicate with the hypervisor, 
too. Maybe we should BUG() here to terminate further processing?

Thanks,
Tom
>

On 4/14/20 1:04 PM, Tom Lendacky wrote:
>> set_memory_decrypted needs to check the return value. I see it
>> consistently return ENOMEM. I've traced that back to
split_large_page
>> in arch/x86/mm/pat/set_memory.c.
> 
> At that point the guest won't be able to communicate with the
> hypervisor, too. Maybe we should BUG() here to terminate further
> processing?
Escalating an -ENOMEM into a crashed kernel seems a bit extreme.
Granted, the guest may be in an unrecoverable state, but the host
doesn't need to be too.

On 4/14/20 3:12 PM, Dave Hansen wrote:
> On 4/14/20 1:04 PM, Tom Lendacky wrote:
>>> set_memory_decrypted needs to check the return value. I see it
>>> consistently return ENOMEM. I've traced that back to
split_large_page
>>> in arch/x86/mm/pat/set_memory.c.
>>
>> At that point the guest won't be able to communicate with the
>> hypervisor, too. Maybe we should BUG() here to terminate further
>> processing?
> 
> Escalating an -ENOMEM into a crashed kernel seems a bit extreme.
> Granted, the guest may be in an unrecoverable state, but the host
> doesn't need to be too.
> 
The host wouldn't be. This only happens in a guest, so it would be just 
causing the guest kernel to panic early in the boot.

Thanks,
Tom

On Tue, Apr 14, 2020 at 03:04:42PM -0500, Tom Lendacky
wrote:
> At that point the guest won't be able to communicate with the
hypervisor,
> too. Maybe we should BUG() here to terminate further processing?
We could talk to the hypervisor, there is still the boot-GHCB in the
bss-decrypted section. But there is nothing that could be done here
anyway besides terminating the guest.


Regards,

	Joerg