linux-kernel - Re: [PATCH 1/2] KVM: SVM: Convert plain error code numbers to defines

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Message-ID: <Z2DIrxpwg1dUdm3y@google.com>
Date: Mon, 16 Dec 2024 16:41:19 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Melody Wang <huibo.wang@....com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, KVM <kvm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Tom Lendacky <thomas.lendacky@....com>, 
	Dhaval Giani <dhaval.giani@....com>, Pavan Kumar Paluri <papaluri@....com>
Subject: Re: [PATCH 1/2] KVM: SVM: Convert plain error code numbers to defines

On Fri, Dec 06, 2024, Melody Wang wrote:
> Convert VMGEXIT SW_EXITINFO1 codes from plain numbers to proper defines.
> 
> No functionality changed.
> 
> Signed-off-by: Melody Wang <huibo.wang@....com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> Reviewed-by: Pavan Kumar Paluri <papaluri@....com>
> ---
>  arch/x86/include/asm/sev-common.h |  8 ++++++++
>  arch/x86/kvm/svm/sev.c            | 12 ++++++------
>  arch/x86/kvm/svm/svm.c            |  2 +-
>  3 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
> index 98726c2b04f8..01d4744e880a 100644
> --- a/arch/x86/include/asm/sev-common.h
> +++ b/arch/x86/include/asm/sev-common.h
> @@ -209,6 +209,14 @@ struct snp_psc_desc {
>  
>  #define GHCB_RESP_CODE(v)		((v) & GHCB_MSR_INFO_MASK)
>  
> +/*
> + * Error codes of the GHCB SW_EXITINFO1 related to GHCB input that can be
> + * communicated back to the guest
> + */
> +#define GHCB_HV_RESP_SUCCESS		0

Somewhat of a nit, but I don't think "SUCCESS" is appropriate due to the bizarre
return codes for Page State Change (PSC) requests.  For unknown reasons (really,
why!?!?), PSC requests apparently always get back '0', but then put a bunch of
errors into SW_EXITINFO2, including cases that are clearly not "success".

FWIW, "no action" isn't much better, because it too directly conflicts with
the documentation for PSC:

  The page state change request was interrupted, retry the request.
                                                 ^^^^^^^^^^^^^^^^^
I'm all for having svm_vmgexit_success(), but I think the macro needs to be
NO_ACTION (even though it too is flawed), because I strongly suspect that patch 2
deliberately avoided SUCCESS in snp_handle_guest_req() and snp_complete_psc()
specifically because you knew SUCCESS would be misleading.

> +#define GHCB_HV_RESP_ISSUE_EXCEPTION	1
> +#define GHCB_HV_RESP_MALFORMED_INPUT	2

Where is '2' actually documented?  I looked all over the GHCB and the only ones
I can find are '0' and '1'.

  0x0000
    o No action requested by the hypervisor.
  0x0001
    o The hypervisor has requested an exception be issued

And again, somewhat of a nit, but PSC ruins all the fun once more, because it
quite clearly has multiple "malformed input" responses.  So if PSC can get rejected
with "bad input", why on earth would it not use GHCB_HV_RESP_MALFORMED_INPUT?

  o SW_EXITINFO2[31:0] == 0x00000001
    The page_state_change_header structure is not valid

  o SW_EXITINFO2[31:0] == 0x00000002
    The page_state_change_entry structure, identified by
    page_state_change_header.cur_entry, is not valid.