linux-kernel - Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

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Message-ID: <20180906203926.GA5717@linux.intel.com>
Date:   Thu, 6 Sep 2018 13:39:26 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     Borislav Petkov <bp@...e.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active

On Thu, Sep 06, 2018 at 03:20:46PM -0500, Brijesh Singh wrote:
> 
> 
> On 09/06/2018 02:47 PM, Sean Christopherson wrote:
> ...
> 
> >>
> >>Yes, the auxiliary array will dumped into the regular .bss when
> >>CONFIG_AMD_MEM_ENCRYPT=n. Typically it will be few k, I am not
> >>sure if its worth complicating the code to save those extra memory.
> >>Most of the distro's have CONFIG_AMD_MEM_ENCRYPT=y anyways.
> >
> >I just realized that we'll try to create a bogus array if 'NR_CPUS <=
> >HVC_BOOT_ARRAY_SIZE'.  A bit ugly, but we could #ifdef away both that
> >and CONFIG_AMD_MEM_ENCRYPT=n in a single shot, e.g.:
> >
> >#if defined(CONFIG_AMD_MEM_ENCRYPT) && NR_CPUS > HVC_BOOT_ARRAY_SIZE
> >#define HVC_AUX_ARRAY_SIZE  \
> >	PAGE_ALIGN((NR_CPUS - HVC_BOOT_ARRAY_SIZE) * \
> >		   sizeof(struct pvclock_vsyscall_time_info))
> >static struct pvclock_vsyscall_time_info
> >	hv_clock_aux[HVC_AUX_ARRAY_SIZE] __decrypted __aligned(PAGE_SIZE);
> >#endif
> >
> 
> The HVC_BOOT_ARRAY_SIZE macro uses sizeof(..) and to my understanding
> the sizeof operators are not allowed in '#if'. Anyway, I will try to see
> if it can be used, if not then I will stick to CONFIG_AMD_MEM_ENCRYPT
> check.

Hmm, we'll need something otherwise 'NR_CPUS - HVC_BOOT_ARRAY_SIZE'
will wrap and cause build errors.