linux-kernel - Re: [PATCH kernel v2] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest

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Message-ID: <e009f70e-32fc-404a-8560-f059b8999895@amd.com>
Date:   Sun, 1 Oct 2023 20:40:30 +1100
From:   Alexey Kardashevskiy <aik@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH kernel v2] x86/compressed/64: reduce #VC nesting for
 intercepted CPUID for SEV-SNP guest


On 30/9/23 17:17, Borislav Petkov wrote:
> On Tue, Sep 26, 2023 at 02:05:26PM +1000, Alexey Kardashevskiy wrote:
>> vc_raw_handle_exception #1: exit_code 72 (CPUID) eax d ecx 1
>> We lock the main GHCB and while it is locked we get to
> 
> Please use passive voice in your commit message: no "we" or "I", etc,
> and describe your changes in imperative mood.
> 
> Also, pls read section "2) Describe your changes" in
> Documentation/process/submitting-patches.rst for more details.
> 
> Also, see section "Changelog" in
> Documentation/process/maintainer-tip.rst
> 
> Bottom line is: personal pronouns are ambiguous in text, especially with
> so many parties/companies/etc developing the kernel so let's avoid them
> please.
> 
>> snp_cpuid_postprocess() which executes "rdmsr" of MSR_IA32_XSS==0xda0 which
>> triggers:
>>
>> vc_raw_handle_exception #2: exit_code 7c (MSR) ecx da0
>> Here we lock the backup ghcb.
>>
>> And then PMC NMI comes which cannot complete as there is no GHCB page left
>> to use:
>>
>> CPU: 5 PID: 566 Comm: touch Not tainted 6.5.0-rc2-aik-ad9c-g7413e71d3dcf-dirty #27
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown unknown
>> Call Trace:
>>   <NMI>
>>   dump_stack_lvl+0x44/0x60
>>   panic+0x222/0x310
>>   ____sev_get_ghcb+0x21e/0x220
>>   __sev_es_nmi_complete+0x28/0xf0
>>   exc_nmi+0x1ac/0x1c0
>>   end_repeat_nmi+0x16/0x67
>> ...
>>   </NMI>
>>   <TASK>
>>   vc_raw_handle_exception+0x9e/0x2c0
>>   kernel_exc_vmm_communication+0x4d/0xa0
>>   asm_exc_vmm_communication+0x31/0x60
>> RIP: 0010:snp_cpuid+0x2ad/0x420
> 
> Drop that splat like we talked.
> 
>> +/* Paravirt SEV-ES rdmsr which avoids extra #VC event */
>> +#define rdmsr_safe_GHCB(msr, low, high, ghcb, ctxt) ({				\
>> +	int __ret;								\
>> +										\
>> +	ghcb_set_rcx((ghcb), (msr));						\
>> +	__ret = sev_es_ghcb_hv_call((ghcb), (ctxt), SVM_EXIT_MSR, 0, 0);	\
>> +	if (__ret == ES_OK) {							\
>> +		low  = (ghcb)->save.rax;					\
>> +		high = (ghcb)->save.rdx;					\
>> +		/* Invalidate qwords for likely another following GHCB call */	\
>> +		vc_ghcb_invalidate(ghcb);					\
>> +	}									\
>> +	__ret; })
>> +
> 
> First of all, this should be a function, not a macro.

Ingo says different, who wins? :)

> Then, it should be defined only in sev-shared.c for now.

sev-shared.c makes me sad. Including .c is not ... nice, I would avoid 
adding stuff to it at any cost.

> Furthermore, it should not be called "rdmsr" or so but something like
> 
> ghcb_prot_read_msr()
> 
> or so to denote that it is using the GHCB protocol to read the MSR. I'm
> sure it'll gain more users with time.

What is "prot" going to signify?

And what about Tom's "x86/sev: Fix SNP CPUID requests to the 
hypervisor", are you taking that one or I have to repost this one and 
the Tom's patch?


-- 
Alexey