linux-kernel - RE: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages

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Date:   Fri, 20 Mar 2020 17:22:16 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Marc Zyngier <maz@...nel.org>, gregkh <gregkh@...uxfoundation.org>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        Andy Whitcroft <apw@...onical.com>,
        vkuznets <vkuznets@...hat.com>, Jason Wang <jasowang@...hat.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: RE: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions
 for Hyper-V size pages

From: Arnd Bergmann <arnd@...db.de> Sent: Friday, March 20, 2020 9:28 AM
> 
> On Thu, Mar 19, 2020 at 10:43 PM Michael Kelley <mikelley@...rosoft.com> wrote:
> > From: Arnd Bergmann <arnd@...db.de> Sent: Wednesday, March 18, 2020 2:58 AM
> > > On Wed, Mar 18, 2020 at 1:15 AM Michael Kelley <mikelley@...rosoft.com> wrote:
> > > My point was to keep the functions but use alloc_pages() internally,
> > > so you can deal with the hypervisor having a larger page size than
> > > the guest, which seems to be a more important scenario if I correctly
> > > understand the differences between the way Windows and Linux
> > > deal with page cache.
> >
> > OK, I see now what you are getting at.  I should spell out my
> > assumption, which is the opposite.  Hyper-V won't have a page
> > size other than 4K anytime in the foreseeable future.  The
> > code is too wedded to the x86 4K page size, and the host-guest
> > interfaces have a lot of implicit assumptions that the page size is
> > 4K (unfortunately).  But the last time I looked, RHEL for ARM64 is
> > delivered with a 64K page size.  So my assumption is that the only
> > combination that really matters is the guest page size being larger
> > than the Hyper-V page size.  The other way around just won't
> > happen without a major overhaul to Hyper-V, including a rework
> > of the guest-host interface.
> 
> Ok, got it. We should really ask Red Hat to change the page size,
> but as long as you care existing systems, and you expect this to
> result in gigabytes of allocation on future systems, having the
> wrapper seems reasonable.
> 
> Maybe you could fall back to alloc_page when PAGE_SIZE equals
> HV_HYP_PAGE_SIZE though? I'm not sure what the tradeoff
> between kmalloc and alloc_page is these days, other than the
> feeling that alloc_page is the more obvious choice when you know
> you always want exactly a page here.
> 

Yes, that works for me.

Michael