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Re: [Qemu-devel] [PATCH 0/7] VNVRAM persistent storage

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 0/7] VNVRAM persistent storage
Date: Fri, 24 May 2013 11:59:38 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, May 23, 2013 at 01:44:40PM -0400, Corey Bryant wrote:
> This patch series provides VNVRAM persistent storage support that
> QEMU can use internally.  The initial target user will be a software
> vTPM 1.2 backend that needs to store keys in VNVRAM and be able to
> reboot/migrate and retain the keys.
> This support uses QEMU's block driver to provide persistent storage
> by reading/writing VNVRAM data from/to a drive image.  The VNVRAM
> drive image is provided with the -drive command line option just like
> any other drive image and the vnvram_create() API will find it.
> The APIs allow for VNVRAM entries to be registered, one at a time,
> each with a maximum blob size.  Entry blobs can then be read/written
> from/to an entry on the drive.  Here's an example of usage:
> VNVRAM *vnvram;
> int errcode
> const VNVRAMEntryName entry_name;
> const char *blob_w = "blob data";
> char *blob_r;
> uint32_t blob_r_size;
> vnvram = vnvram_create("drive-ide0-0-0", false, &errcode);
> strcpy((char *)entry_name, "first-entry");

VNVRAMEntryName is very prone to buffer overflow.  I hope real code
doesn't use strcpy().  The cast is ugly, please don't hide the type.

> vnvram_register_entry(vnvram, &entry_name, 1024);
> vnvram_write_entry(vnvram, &entry_name, (char *)blob_w, strlen(blob_w)+1);
> vnvram_read_entry(vnvram, &entry_name, &blob_r, &blob_r_size);

These are synchronous functions.  If I/O is involved then this is a
problem: QEMU will be blocked waiting for host I/O to complete and the
big QEMU lock is held.  This can cause poor guest interactivity and poor
scalability because vcpus cannot make progress, neither can the QEMU
monitor respond.


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